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I didn't find a trivial way to get the time offset in minutes between the local time and the UTC time.
At first I intended to use tzset() but it doesn't provide the daylight saving time. According to the man page, it is simply an integer different of zero if day light saving is in effect. While it is usually an hour, it may be half an hour in some country.
I would prefer avoiding to compute the time difference between current UTC returned by gmtime() and localtime().
A more general solution would give me this information for a specified location and a positive time_t value, or at least locally.
Edit 1: the use case is to get the right local time offset for https://github.com/chmike/timez.
BTW, If you thought libc functions to manipulate time were Ok, read this https://rachelbythebay.com/w/2013/03/17/time/.
Edit 2: the best and simplest solution I have so far to compute the time offset to UTC in minutes is
// Bogus: assumes DST is always one hour
tzset();
int offset = (int)(-timezone / 60 + (daylight ? 60 : 0));
The problem is to determine the real day light saving time.
Edit 3: Inspired by the answer of #trenki, I came up with the following solution. This is a hack in that it tricks mktime() to consider the output of gmtime() as the localtime. The result is inaccurate when the DST change is in the time span between UTC time and localtime.
#include <stdio.h>
#include <time.h>
int main()
{
time_t rawtime = time(NULL);
struct tm *ptm = gmtime(&rawtime);
// Request that mktime() looksup dst in timezone database
ptm->tm_isdst = -1;
time_t gmt = mktime(ptm);
double offset = difftime(rawtime, gmt) / 60;
printf("%f\n", offset);
return 0;
}
This C code computes the local time offset in minutes relative to UTC. It assumes that DST is always one hour offset.
#include <stdio.h>
#include <time.h>
int main()
{
time_t rawtime = time(NULL);
struct tm *ptm = gmtime(&rawtime);
time_t gmt = mktime(ptm);
ptm = localtime(&rawtime);
time_t offset = rawtime - gmt + (ptm->tm_isdst ? 3600 : 0);
printf("%i\n", (int)offset);
}
It uses gmtime and localtime though. Why don't you want to use those functions?
Does your system's strftime() function support the %z and %Z specifiers? On FreeBSD,
%Z is replaced by the time zone name.
%z is replaced by the time zone offset from UTC; a leading plus sign
stands for east of UTC, a minus sign for west of UTC, hours and
minutes follow with two digits each and no delimiter between them
(common form for RFC 822 date headers).
and I can use this to print this:
$ date +"%Z: %z"
CEST: +0200
ISO C99 has this in 7.23.3.5 The strftime function:
%z is replaced by the offset from UTC in the ISO 8601 format
‘‘−0430’’ (meaning 4 hours 30 minutes behind UTC, west of Greenwich),
or by no characters if no time zone is determinable. [tm_isdst]
%Z is replaced by the locale’s time zone name or abbreviation, or by no
characters if no time zone is determinable. [tm_isdst]
... to get local time offset ... relative to UTC?
#Serge Ballesta answer is good. So I though I would test it and clean-up a few details. I would have posted as a comment but obviously too big for that. I only exercised it for my timezone, but though others may want to try on their machine and zone.
I made to community wiki as not to garner rep. Imitation is the sincerest form of flattery
This answer is akin to #trenki except that it subtracts nearby struct tm values instead of assuming DST shift is 1 hour and time_t is in seconds.
#include <limits.h>
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
// return difference in **seconds** of the tm_mday, tm_hour, tm_min, tm_sec members.
long tz_offset_second(time_t t) {
struct tm local = *localtime(&t);
struct tm utc = *gmtime(&t);
long diff = ((local.tm_hour - utc.tm_hour) * 60 + (local.tm_min - utc.tm_min))
* 60L + (local.tm_sec - utc.tm_sec);
int delta_day = local.tm_mday - utc.tm_mday;
// If |delta_day| > 1, then end-of-month wrap
if ((delta_day == 1) || (delta_day < -1)) {
diff += 24L * 60 * 60;
} else if ((delta_day == -1) || (delta_day > 1)) {
diff -= 24L * 60 * 60;
}
return diff;
}
void testtz(void) {
long off = -1;
int delta = 600;
for (time_t t = 0; t < LONG_MAX-delta; t+=delta) {
long off2 = tz_offset_second(t);
// Print time whenever offset changes.
if (off != off2) {
struct tm utc = *gmtime(&t);
printf("%10jd %04d-%02d-%02dT%02d:%02d:%02dZ\n", (intmax_t) t,
utc.tm_year + 1900, utc.tm_mon + 1, utc.tm_mday,
utc.tm_hour, utc.tm_min, utc.tm_sec);
struct tm local = *localtime(&t);
off = off2;
printf("%10s %04d-%02d-%02d %02d:%02d:%02d %2d %6ld\n\n", "",
local.tm_year + 1900, local.tm_mon + 1, local.tm_mday,
local.tm_hour, local.tm_min, local.tm_sec, local.tm_isdst ,off);
fflush(stdout);
}
}
puts("Done");
}
Output
v----v Difference in seconds
0 1970-01-01T00:00:00Z
1969-12-31 18:00:00 0 -21600
5731200 1970-03-08T08:00:00Z
1970-03-08 03:00:00 1 -18000
26290800 1970-11-01T07:00:00Z
1970-11-01 01:00:00 0 -21600
...
2109222000 2036-11-02T07:00:00Z
2036-11-02 01:00:00 0 -21600
2120112000 2037-03-08T08:00:00Z
2037-03-08 03:00:00 1 -18000
2140671600 2037-11-01T07:00:00Z
2037-11-01 01:00:00 0 -21600
Done
IMHO the only foolproof and portable way is to use localtime and gmtime and manually compute the delta in minute because those 2 functions exist on all known systems. For example:
int deltam() {
time_t t = time(NULL);
struct tm *loc = localtime(&t);
/* save values because they could be erased by the call to gmtime */
int loc_min = loc->tm_min;
int loc_hour = loc->tm_hour;
int loc_day = loc->tm_mday;
struct tm *utc = gmtime(&t);
int delta = loc_min - utc->tm_min;
int deltaj = loc_day - utc->tm_mday;
delta += (loc_hour - utc->tm_hour) * 60;
/* hack for the day because the difference actually is only 0, 1 or -1 */
if ((deltaj == 1) || (deltaj < -1)) {
delta += 1440;
}
else if ((deltaj == -1) || (deltaj > 1)) {
delta -= 1440;
}
return delta;
}
Beware, I did not test all possible corner cases, but it could be a starting point for your requirement.
I would like to submit yet another answer to this question, one that AFAICS also deals with the IDL.
This solution depends on timegm and mktime. On Windows timegm is available as _mkgmtime from the CRT, in other words define a conditional macro.
#if _WIN32
# define timegm _mkgmtime
#endif
int local_utc_offset_minutes ( ) {
time_t t = time ( NULL );
struct tm * locg = localtime ( &t );
struct tm locl;
memcpy ( &locl, locg, sizeof ( struct tm ) );
return (int)( timegm ( locg ) - mktime ( &locl ) ) / 60;
}
Here is my way:
time_t z = 0;
struct tm * pdt = gmtime(&z);
time_t tzlag = mktime(pdt);
Alternative with automatic, local storage of struct tm:
struct tm dt;
memset(&dt, 0, sizeof(struct tm));
dt.tm_mday=1; dt.tm_year=70;
time_t tzlag = mktime(&dt);
tzlag, in seconds, will be the negative of the UTC offset; lag of your timezone Standard Time compared to UTC:
LocalST + tzlag = UTC
If you want to also account for "Daylight savings", subtract tm_isdst from tzlag, where tm_isdst is the field for a particular local time struct tm, after applying mktime to it (or after obtaining it with localtime ).
Why it works:
The set struct tm is for "epoch" moment, Jan 1 1970, which corresponds to a time_t of 0.
Calling mktime() on that date converts it to time_t as if it were UTC (thus getting 0), then subtracts the UTC offset from it in order to produce the output time_t. Thus it produces negative of UTC_offset.
How do I do the above? There is mktime function but that treats the input as expressed in local time but how do i perform the conversion if my input tm variable happens to be in UTC.
Use timegm() instead of mktime()
for those on windows, the below function is available:
_mkgmtime
link for more info: https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/mkgmtime-mkgmtime32-mkgmtime64
Here is a solution I use (Can't recall where I found it) when it isn't a windows platform
time_t _mkgmtime(const struct tm *tm)
{
// Month-to-day offset for non-leap-years.
static const int month_day[12] =
{0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334};
// Most of the calculation is easy; leap years are the main difficulty.
int month = tm->tm_mon % 12;
int year = tm->tm_year + tm->tm_mon / 12;
if (month < 0) { // Negative values % 12 are still negative.
month += 12;
--year;
}
// This is the number of Februaries since 1900.
const int year_for_leap = (month > 1) ? year + 1 : year;
time_t rt = tm->tm_sec // Seconds
+ 60 * (tm->tm_min // Minute = 60 seconds
+ 60 * (tm->tm_hour // Hour = 60 minutes
+ 24 * (month_day[month] + tm->tm_mday - 1 // Day = 24 hours
+ 365 * (year - 70) // Year = 365 days
+ (year_for_leap - 69) / 4 // Every 4 years is leap...
- (year_for_leap - 1) / 100 // Except centuries...
+ (year_for_leap + 299) / 400))); // Except 400s.
return rt < 0 ? -1 : rt;
}
The answer of Loki Astari was a good start, timegm is one of the possible solutions. However, the man page of timegm gives a portable version of it, as timegm is not POSIX-compliant. Here it is:
#include <time.h>
#include <stdlib.h>
time_t
my_timegm(struct tm *tm)
{
time_t ret;
char *tz;
tz = getenv("TZ");
if (tz)
tz = strdup(tz);
setenv("TZ", "", 1);
tzset();
ret = mktime(tm);
if (tz) {
setenv("TZ", tz, 1);
free(tz);
} else
unsetenv("TZ");
tzset();
return ret;
}
timegm() works, but is not present on all systems.
Here's a version that only uses ANSI C. (EDIT: not strictly ANSI C! I'm doing math on time_t, assuming that the units are in seconds since the epoch. AFAIK, the standard does not define the units of time_t.) Note, it makes use of a hack, so-to-speak, to determine the machine's time zone and then adjusts the result from mktime accordingly.
/*
returns the utc timezone offset
(e.g. -8 hours for PST)
*/
int get_utc_offset() {
time_t zero = 24*60*60L;
struct tm * timeptr;
int gmtime_hours;
/* get the local time for Jan 2, 1900 00:00 UTC */
timeptr = localtime( &zero );
gmtime_hours = timeptr->tm_hour;
/* if the local time is the "day before" the UTC, subtract 24 hours
from the hours to get the UTC offset */
if( timeptr->tm_mday < 2 )
gmtime_hours -= 24;
return gmtime_hours;
}
/*
the utc analogue of mktime,
(much like timegm on some systems)
*/
time_t tm_to_time_t_utc( struct tm * timeptr ) {
/* gets the epoch time relative to the local time zone,
and then adds the appropriate number of seconds to make it UTC */
return mktime( timeptr ) + get_utc_offset() * 3600;
}
The following implementation of timegm(1) works swimmingly on Android, and probably works on other Unix variants as well:
time_t timegm( struct tm *tm ) {
time_t t = mktime( tm );
return t + localtime( &t )->tm_gmtoff;
}
POSIX page for tzset, describes global variable extern long timezone which contains the local timezone as an offset of seconds from UTC. This will be present on all POSIX compliant systems.
In order for timezone to contain the correct value, you will likely need to call tzset() during your program's initialization.
You can then just subtract timezone from the output of mktime to get the output in UTC.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
time_t utc_mktime(struct tm *t)
{
return (mktime(t) - timezone) - ((t->tm_isdst > 0) * 3600);
}
int main(int argc, char **argv)
{
struct tm t = { 0 };
tzset();
utc_mktime(&t);
}
Note: Technically tzset() and mktime() aren't guaranteed to be threadsafe.
If a thread accesses tzname, [XSI] [Option Start] daylight, or timezone [Option End] directly while another thread is in a call to tzset(), or to any function that is required or allowed to set timezone information as if by calling tzset(), the behavior is undefined.
...but the majority of implementations are. GNU C uses mutexes in tzset() to avoid concurrent modifications to the global variables it sets, and mktime() sees very wide use in threaded programs without synchronization. I suspect if one were to encounter side effects, it would be from using setenv() to alter the value of TZ as done in the answer from #liberforce.
I was troubled by the issue of mktime() as well. My solution is the following
time_t myTimegm(std::tm * utcTime)
{
static std::tm tmv0 = {0, 0, 0, 1, 0, 80, 0, 0, 0}; //1 Jan 1980
static time_t utcDiff = std::mktime(&tmv0) - 315532801;
return std::mktime(utcTime) - utcDiff;
}
The idea is to get the time difference by calling std::mktime() with a known time (in this case 1980/01/01) and subtract its timestamp (315532801). Hope it helps.
Here's my take, which is based exclusively on time_t/tm conversion functions, and the only presumption it makes about time_t is that it is linear:
Pretending against better knowledge the tm structure holds local time (non-DST if anyone asks; it doesn't matter, but must be consistent with step 3), convert it to time_t.
Convert the date back into a tm structure, but this time in UTC representation.
Pretending against better knowledge that tm structure to also hold local (non-DST if anyone asks, but more importantly consistent with step 1), and convert it to time_t once more.
From the two time_t results I can now compute the difference between local time (non-DST if anyone asks) and UTC in time_t units.
Adding that difference to the first time_t result gives me the proper time in UTC.
Note that computation of the difference can conceivably be done once, and then applied later to as many dates as desired; this might be a way to solve issues arising from the lack of thread-safety in gmtime.
(Edit: Then again, this might cause issues if the time zone is changed between the date used to compute the offset, and the date to be converted.)
tm tt;
// populate tt here
tt.tm_isdst = 0;
time_t tLoc = mktime(&tt);
tt = *gmtime(&tLoc);
tt.tm_isdst = 0;
time_t tRev = mktime(&tt);
time_t tDiff = tLoc - tRev;
time_t tUTC = tLoc + tDiff;
Caveat: If the system uses a TAI-based time_t (or anything else that does respect leap seconds), the resulting time may be off by 1 second if applied to a point in time close to a leap second insertion.
This is really a comment with code to address the answer by Leo Accend:
Try the following:
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
/*
* A bit of a hack that lets you pull DST from your Linux box
*/
time_t timegm( struct tm *tm ) { // From Leo's post, above
time_t t = mktime( tm );
return t + localtime( &t )->tm_gmtoff;
}
main()
{
struct timespec tspec = {0};
struct tm tm_struct = {0};
if (gettimeofday(&tspec, NULL) == 0) // clock_gettime() is better but not always avail
{
tzset(); // Not guaranteed to be called during gmtime_r; acquire timezone info
if (gmtime_r(&(tspec.tv_sec), &tm_struct) == &tm_struct)
{
printf("time represented by original utc time_t: %s\n", asctime(&tm_struct));
// Go backwards from the tm_struct to a time, to pull DST offset.
time_t newtime = timegm (&tm_struct);
if (newtime != tspec.tv_sec) // DST offset detected
{
printf("time represented by new time_t: %s\n", asctime(&tm_struct));
double diff = difftime(newtime, tspec.tv_sec);
printf("DST offset is %g (%f hours)\n", diff, diff / 3600);
time_t intdiff = (time_t) diff;
printf("This amounts to %s\n", asctime(gmtime(&intdiff)));
}
}
}
exit(0);
}
For all timezones and at all times would be exceedingly difficult if not impossible. You would need an accurate record of all the various arbitrary timezone and daylight savings time (DST) decrees. Sometimes, it is not clear who the local authority is, never mind what was decreed and when. Most systems, for example, are off by one second for uptime (time system has been up) or boottime (timestamp system booted), if a leap second was spanned. A good test would be a date that was once in DST but now is not (or vis versa). (It was not too long ago in the US that it changed.)
It's a simple question, but the solution appears to be far from simple. I would like to know how to convert from UTC to local time. I am looking for a solution in C that's standard and more or less guaranteed to work on any computer at any location.
I have read the following links carefully but I can't find a solution there:
Converting string containing localtime into UTC in C
Converting Between Local Times and GMT/UTC in C/C++
I have tried a number of variations, such as (datetime is a string with time and date in UTC):
strptime(datetime, "%A %B %d %Y %H %M %S", tp);
strftime(printtime, strlen(datetime), "%A %B %d %Y %H %M %S", tp);
Or
strptime(datetime, "%A %B %d %Y %H %M %S", tp);
lt=mktime(tp);
printtime=ctime(<);
No matter what I try printtime ends up being the same as UTC.
Edit 11-29-2013: based on the very helpful answer by "R" below I finally got around to create a working example. I found it to be working correct in the two timezones I tested it, CET and PST:
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
long long diff_tm(struct tm *a, struct tm *b)
{
return a->tm_sec - b->tm_sec
+60LL*(a->tm_min - b->tm_min)
+3600LL*(a->tm_hour - b->tm_hour)
+86400LL*(a->tm_yday - b->tm_yday)
+(a->tm_year-70)*31536000LL
-(a->tm_year-69)/4*86400LL
+(a->tm_year-1)/100*86400LL
-(a->tm_year+299)/400*86400LL
-(b->tm_year-70)*31536000LL
+(b->tm_year-69)/4*86400LL
-(b->tm_year-1)/100*86400LL
+(b->tm_year+299)/400*86400LL;
}
int main()
{
time_t utc, local;
char buf[100];
const char datetime[]="2013 11 30 23 30 26 UTC"; /* hard coded date and time in UTC */
struct tm *tp=malloc(sizeof(struct tm));
if(tp==NULL)
exit(-1);
struct tm *localt=malloc(sizeof(struct tm));
if(localt==NULL)
exit(-1);
memset(tp, 0, sizeof(struct tm));
memset(localt, 0, sizeof(struct tm));
printf("UTC date and time to be converted in local time: %s\n", datetime);
/* put values of datetime into time structure *tp */
strptime(datetime, "%Y %m %d %H %M %S %z", tp);
/* get seconds since EPOCH for this time */
utc=mktime(tp);
printf("UTC date and time in seconds since EPOCH: %d\n", utc);
/* lets convert this UTC date and time to local date and time */
struct tm e0={ .tm_year = 70, .tm_mday = 1 }, e1, new;
/* get time_t EPOCH value for e0 (Jan. 1, 1970) */
time_t pseudo=mktime(&e0);
/* get gmtime for this value */
e1=*gmtime(&pseudo);
/* calculate local time in seconds since EPOCH */
e0.tm_sec += utc - diff_tm(&e1, &e0);
/* assign to local, this can all can be coded shorter but I attempted to increase clarity */
local=e0.tm_sec;
printf("local date and time in seconds since EPOCH: %d\n", local);
/* convert seconds since EPOCH for local time into localt time structure */
localt=localtime(&local);
/* get nicely formatted human readable time */
strftime(buf, sizeof buf, "%Y-%m-%d %H:%M:%S %Z", localt);
printf("local date and time: %s\n", buf);
}
It should compile without problems on most systems. I hard coded a time and date in UTC which then will be converted to the local time and date.
If you can assume POSIX (and thus the POSIX specification of time_t as seconds since the epoch), I would first use the POSIX formula to convert to seconds since the epoch:
tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 +
(tm_year-70)*31536000 + ((tm_year-69)/4)*86400 -
((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400
Next, use localtime((time_t []){0}) to get a struct tm representing the epoch in local time. Add the seconds since the epoch to the tm_sec field of this struct tm, then call mktime to canonicalize it.
Edit: Actually the only POSIX dependency is having a known epoch which (time_t)0 corresponds to. Perhaps you can find a way around that if you really need to... for instance using calls to both gmtime and localtime at time_t 0..
Edit 2: A sketch of how to do this:
#include <time.h>
#include <stdio.h>
long long diff_tm(struct tm *a, struct tm *b)
{
return a->tm_sec - b->tm_sec
+60LL*(a->tm_min - b->tm_min)
+3600LL*(a->tm_hour - b->tm_hour)
+86400LL*(a->tm_yday - b->tm_yday)
+(a->tm_year-70)*31536000LL
-(a->tm_year-69)/4*86400LL
+(a->tm_year-1)/100*86400LL
-(a->tm_year+299)/400*86400LL
-(b->tm_year-70)*31536000LL
+(b->tm_year-69)/4*86400LL
-(b->tm_year-1)/100*86400LL
+(b->tm_year+299)/400*86400LL;
}
int main(int argc, char **argv)
{
char buf[100];
struct tm e0 = { .tm_year = 70, .tm_mday = 1 }, e1, new;
time_t pseudo = mktime(&e0);
e1 = *gmtime(&pseudo);
e0.tm_sec += atoi(argv[1]) - diff_tm(&e1, &e0);
mktime(&e0);
strftime(buf, sizeof buf, "%c", &e0);
puts(buf);
}
Please don't mind the ugly output code. This program takes an argument in the form of "seconds relative to the POSIX epoch" and outputs the resulting time in local time. You can convert any UTC time to seconds since the epoch using the formula I cited above. Note that this code does not in any way depend on POSIX, but it does assume the offset returned by diff_tm combined with the seconds-since-the-epoch value does not overflow int. A fix for this would be to use a long long offset and a loop that keeps adding increments no larger than INT_MAX/2 (or smaller than INT_MIN/2) and calling mktime to renormalize until the offset reaches 0.
Ahm ... I might just be a beginner in C, but I got this working example:
#include <time.h>
#include <stdio.h>
int main(void)
{
time_t abs_ts,loc_ts,gmt_ts;
struct tm loc_time_info,gmt_time_info;
/*Absolute time stamp.*/
time(&abs_ts);
/*Now get once the local time for this time stamp,
**and once the GMT (UTC without summer time) time stamp.*/
localtime_r(&abs_ts,&loc_time_info);
gmtime_r(&abs_ts,&gmt_time_info);
/*Convert them back.*/
loc_ts=mktime(&loc_time_info);
gmt_ts=mktime(&gmt_time_info);
/*Unfortunately, GMT still has summer time. Get rid of it:*/
if(gmt_time_info.tm_isdst==1)
{gmt_ts-=3600;}
printf("Local timestamp: %lu\n"
"UTC timestamp: %lu\n"
"Difference in hours: %lu\n\n",
loc_ts,
gmt_ts,
(loc_ts-gmt_ts)/3600);
return 0;
}
Which produces this output:
Local timestamp: 1412554119
GMT timestamp: 1412546919
Difference in hours: 2
Now you have the difference between UTC and local time in seconds. That should be enough to convert it.
One note to your code, aseq: you are using malloc without need here (you can memset values on the stack as well, and malloc can be expensive while stack allocation is often much faster), and you do not free it. That's very, very bad practise.
Another thing:
memset(tp, 0, sizeof(struct tm));
Would be better done if you'd pass sizeof(*tp) (or, if you put tp on the stack, sizeof(tp)) to memset. That ensures that even if the type of your object changes, it will still be fully memset.
To sum-up: the conversion of a broken down date (struct tm) in UTC to a (local) calendar time (time_t) is achieved with timegm() - the opposite of mktime() - BUT timegm() is not a standard function (how logic is that).
The C standard leaves us with only time(), gmtime(), mktime() and difftime().
A workaround found in other docs advises to emulate timegm() by setting first the environment variable TZ to a null string, then calling mktime() resulting in an UTC calendar time, then resetting TZ to its initial value, but once again, this is not standard.
Basically, as I understand it, the difference between a local time and UTC time is just an offset so if we can evaluate that offset, we can adjust the result of mktime(), so here's my proposition:
time_t my_timegm(struct tm *tm) {
time_t epoch = 0;
time_t offset = mktime(gmtime(&epoch));
time_t utc = mktime(tm);
return difftime(utc, offset);
}
A quick test:
int main(void) {
time_t now = time(0);
struct tm local = *localtime(&now);
struct tm utc = *gmtime(&now);
time_t t1 = mktime(&local);
time_t t2 = my_timegm(&utc);
assert(t1 == t2);
printf("t =%lu\nt1=%lu\nt2=%lu\n",now,t1,t2);
return 0;
}
//working stand alone function adjusting UTC to local date and time
//globals(unsigned integers): gps.Mth, gps.Yr, gps.Hm (eg:2115 for 21:15)
//adjust date and time according to UTC
//tz(timezone) eg: 1100, for 11 hours, tzdir: 1 forward, 0 backwards
void AdjustUTCToTimeZone(u16 tz, u8 tzdir){
u8 maxDayInAnyMonth[13] = {0,31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; //gps.Mth 1-12 (not zero)
if(gps.Yr%4==0){maxDayInAnyMonth[2]=29;}//adjust for leapyear
u8 maxDayUtcMth =maxDayInAnyMonth[gps.Mth];
u8 maxDayPrevMth=maxDayInAnyMonth[gps.Mth-1];
if(!maxDayPrevMth){maxDayPrevMth=31;} //month before utc month
u16 hr=(gps.Hm/100)*100;u16 m=gps.Hm-hr; //2115 --> 2100 hr and 15 min
if(tzdir){//adjusting forwards
tz+=gps.Hm;
if(tz>2400){gps.Hm=tz-2400;gps.Day++; //spill over to next day
if(gps.Day>maxDayUtcMth){ gps.Day=1;gps.Mth++; //spill over to next month
if(gps.Mth>12){gps.Mth=1; gps.Yr++; //spill over to next year
}
}
}else{gps.Hm=tz;}
}else{//adjusting backwards
if(tz>gps.Hm){gps.Hm=(2400-(tz-hr))+m;gps.Day--; // back to previous day
if(gps.Day==0){ //back to previous month
gps.Mth--;gps.Day=maxDayPrevMth;
if(!gps.Mth){gps.Mth=12; //back to previous year
gps.Yr--;
}
}
}else{gps.Hm-=tz;}
}
}
I think it's easier than that; time.h defines three variables:
extern int daylight;
extern long timezone;
extern char *tzname[];
which are loaded based on the TZ env variable when you call
tzset();
if you have a utc time in
struct tm date;
date.tm_isdst = 0;
convert it to a time_t using mktime
time_t utc = mktime( &date );
then convert it to local time
time_t local = utc - timezone + ( daylight?3600:0 );
timezone is the number of seconds away from utc for the current timezone and daylight is 1 to indicate daylight savings time is in play and zero for not.
A small caution: When I coded this for a microcontroller and cross compiled, it's time.h defined those variables with initial underscores.
See the man page for time.h
I found that the solution the OP gave did not work in cases when DST applies. For example, in my case, at the current time, DST was not in effect, but if I set the initial date which should convert to local time with DST, then it would not work, i.e. today's date is 3/1/2018 and DST is not in effect, but if I set the date for conversion to, say, 8/1/2018 0:00:00 when DST is in effect, then the solution given would convert to local time, but would not take DST into account. I found that initializing e0 to the date and hour of the initial date/time string and its member tm_isdst to -1 solved the problem. I then created the following program with complementary functions which you can include in your code. The initial format of the date and time is the same that MySQL uses, because I needed it for such purposes.
#include <stdio.h>
#include <time.h>
#include <string.h>
long long diff_tm(struct tm *a, struct tm *b) {
return a->tm_sec - b->tm_sec
+ 60LL * (a->tm_min - b->tm_min)
+ 3600LL * (a->tm_hour - b->tm_hour)
+ 86400LL * (a->tm_yday - b->tm_yday)
+ (a->tm_year - 70) * 31536000LL
- (a->tm_year - 69) / 4 * 86400LL
+ (a->tm_year - 1) / 100 * 86400LL
- (a->tm_year + 299) / 400 * 86400LL
- (b->tm_year - 70) * 31536000LL
+ (b->tm_year - 69) / 4 * 86400LL
- (b->tm_year - 1) / 100 * 86400LL
+ (b->tm_year + 299) /400 * 86400LL;
}
void localToUTC(char *buf, const char *localTime) {
struct tm tp;
strptime(localTime, "%Y-%m-%d %H:%M:%S", &tp);
tp.tm_isdst = -1;
time_t utc = mktime(&tp);
struct tm res = *gmtime(&utc);
strftime(buf, 20, "%Y-%m-%d %H:%M:%S", &res);
}
void utcToLocal(char *buf, const char *utcTime) {
struct tm tp;
strptime(utcTime, "%Y-%m-%d %H:%M:%S", &tp);
tp.tm_isdst = -1;
time_t utc = mktime(&tp);
struct tm e0 = { .tm_year = tp.tm_year, .tm_mday = tp.tm_mday, .tm_mon = tp.tm_mon, .tm_hour = tp.tm_hour, .tm_isdst = -1 };
time_t pseudo = mktime(&e0);
struct tm e1 = *gmtime(&pseudo);
e0.tm_sec += utc - diff_tm(&e1, &e0);
time_t local = e0.tm_sec;
struct tm localt = *localtime(&local);
strftime(buf, 20, "%Y-%m-%d %H:%M:%S", &localt);
}
int main(void) {
char mytime_1[20] = "2018-02-28 13:00:00";
char utctime_1[20], back_1[20];
localToUTC(utctime_1, mytime_1);
utcToLocal(back_1, utctime_1);
printf("My time: %s\n", mytime_1);
printf("UTC time: %s\n", utctime_1);
printf("Back: %s\n", back_1);
printf("-------------------------------------------\n");
char mytime_2[20] = "2018-07-28 17:00:00";
char utctime_2[20], back_2[20];
localToUTC(utctime_2, mytime_2);
utcToLocal(back_2, utctime_2);
printf("My time: %s\n", mytime_2);
printf("UTC time: %s\n", utctime_2);
printf("Back: %s\n", back_2);
printf("-------------------------------------------\n");
return 0;
}
I followed the answer by #Dachschaden and I made an example which also shows human-readable output and I remove the DST option for the difference in seconds between UTC and local time. Here it is:
#include <time.h>
#include <stdio.h>
#define DATE_MAX_STR_SIZE 26
#define DATE_FMT "%FT%TZ%z"
int main() {
time_t now_time, now_time_local;
struct tm now_tm_utc, now_tm_local;
char str_utc[DATE_MAX_STR_SIZE];
char str_local[DATE_MAX_STR_SIZE];
time(&now_time);
gmtime_r(&now_time, &now_tm_utc);
localtime_r(&now_time, &now_tm_local);
/* human readable */
strftime(str_utc, DATE_MAX_STR_SIZE, DATE_FMT, &now_tm_utc);
strftime(str_local, DATE_MAX_STR_SIZE, DATE_FMT, &now_tm_local);
printf("\nUTC: %s", str_utc);
printf("\nLOCAL: %s\n", str_local);
/* seconds (epoch) */
/* let's forget about DST for time difference calculation */
now_tm_local.tm_isdst = 0;
now_tm_utc.tm_isdst = 0;
now_time_local = now_time + (mktime(&now_tm_local) - mktime(&now_tm_utc));
printf("\nUTC in seconds: %lu", now_time);
printf("\nLOCAL in seconds: %lu\n", now_time_local);
return 0;
}
Output on my machine is:
UTC: 2016-05-05T15:39:11Z-0500
LOCAL: 2016-05-05T11:39:11Z-0400
UTC in seconds: 1462462751
LOCAL in seconds: 1462448351
Note that DST is on in this case (there's a 1 hour time zone offset difference between UTC and LOCAL).
try this, test output:
utcEpochTime: 1487652688, localEpochTime: 1487699488, diff: 46800
$ python
>>>46800 / 60 / 60
13
the diff is 13 hours, which is good, as my timezone is UTC+8.
#include <stdio.h>
#include <time.h>
int main(int argc, char *argv[])
{
time_t utcEpochTime = time(0);
time_t localEpochTime = 0;
struct tm tm = {0};
localtime_r(&utcEpochTime, &tm);
tm.tm_isdst = -1;
localEpochTime = timegm(&tm);
printf("utcEpochTime: %d, localEpochTime: %d, diff: %d\n", (int)utcEpochTime, (int)localEpochTime, (int)(localEpochTime - utcEpochTime));
return 0;
}
A simple and effective way: Add (or subtract) the number of seconds between your time zone and UTC (considering daylight saving time).
As an example that worked just fine a minute ago, on December 30, 2017, with U.S. Mountain Standard Time (no DST), which is 7 hours behind UTC:
time_t current_time_UTC;
time_t current_time_MST;
struct tm *current_broken_time_MST;
uint32_t seven_hours_in_seconds = 25200; // Get this any way you want
current_time_UTC = time (NULL); // UTC
current_time_MST = current_time_UTC - seven_hours_in_seconds; // MST
current_broken_time_MST = localtime (¤t_time_MST); // MST
Enjoy.
void CTestDlg::OnBtnTest()
{
HANDLE hFile;
WIN32_FIND_DATA wfd;
SYSTEMTIME systime;
FILETIME localtime;
char stime[32]; //
memset(&wfd, 0, sizeof(wfd));
if((hFile=FindFirstFile( "F:\\VC\\MFC\\Test\\Release\\Test.exe ", &wfd))==INVALID_HANDLE_VALUE)
{
char c[2];
DWORD dw=GetLastError();
wsprintf(c, "%d ", dw);
AfxMessageBox(c);
return ;//
}
FileTimeToLocalFileTime(&wfd.ftLastWriteTime,&localtime);
FileTimeToSystemTime(&localtime,&systime);
sprintf(stime, "%4d-%02d-%02d %02d:%02d:%02d ",
systime.wYear,systime.wMonth,systime.wDay,systime.wHour,
systime.wMinute,systime.wSecond);
AfxMessageBox(stime);
}
I have a time stored in a 64 bit int of the form 20110103101419 (i.e. representing 2011-01-03 10:14:19). How do I convert that to seconds since 1970 ?
My C is a bit rusty, but looking at the other two answers I would write a function as follows, returning the number of seconds since epoch or -1 in case of error.
#include <stdio.h>
#include <time.h>
time_t convertDecimalTime(long dt) {
struct tm time_str;
time_str.tm_isdst = -1;
time_str.tm_sec = dt%100; dt/=100;
time_str.tm_min = dt%100; dt/=100;
time_str.tm_hour = dt%100; dt/=100;
time_str.tm_mday = dt%100; dt/=100;
time_str.tm_mon = dt%100-1; dt/=100;
time_str.tm_year = dt%10000 - 1900;
return mktime(&time_str);
}
Please refer to strftime . You can parse the intput and use %s format to display the number of seconds since the Epoch,
Use mktime to create a struct tm then you can use strftime with %s format parameter to get number of seconds since the epoch.
I have something like this:
char *current_day, *current_time;
system("date +%F");
system("date +%T");
It prints the current day and time in the stdout, but I want to get this output or assign them to the current_day and current_time variables, so that I can do some processing with those values later on.
current_day ==> current day
current_time ==> current time
The only solution that I can think of now is to direct the output to some file, and then read the file and then assign the values of date and time to current_day and current_time. But I think this is not a good way. Is there any other short and elegant way?
Use time() and localtime() to get the time:
#include <stdio.h>
#include <time.h>
int main()
{
time_t t = time(NULL);
struct tm tm = *localtime(&t);
printf("now: %d-%02d-%02d %02d:%02d:%02d\n", tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday, tm.tm_hour, tm.tm_min, tm.tm_sec);
}
strftime (C89)
Martin mentioned it, here's an example:
main.c
#include <assert.h>
#include <stdio.h>
#include <time.h>
int main(void) {
time_t t = time(NULL);
struct tm *tm = localtime(&t);
char s[64];
size_t ret = strftime(s, sizeof(s), "%c", tm);
assert(ret);
printf("%s\n", s);
return 0;
}
GitHub upstream.
Compile and run:
gcc -std=c89 -Wall -Wextra -pedantic -o main.out main.c
./main.out
Sample output:
Thu Apr 14 22:39:03 2016
The %c specifier produces the same format as ctime.
One advantage of this function is that it returns the number of bytes written, allowing for better error control in case the generated string is too long:
RETURN VALUE
Provided that the result string, including the terminating null byte, does not exceed max bytes, strftime() returns the number of bytes (excluding the terminating null byte) placed in the array s. If the length of the result string (including the terminating null byte) would exceed max bytes, then strftime() returns 0, and the contents of the array are undefined.
Note that the return value 0 does not necessarily indicate an error. For example, in many locales %p yields an empty string. An empty format string will likewise yield an empty string.
asctime and ctime (C89, deprecated in POSIX 7)
asctime is a convenient way to format a struct tm:
main.c
#include <stdio.h>
#include <time.h>
int main(void) {
time_t t = time(NULL);
struct tm *tm = localtime(&t);
printf("%s", asctime(tm));
return 0;
}
Sample output:
Wed Jun 10 16:10:32 2015
And there is also ctime() which the standard says is a shortcut for:
asctime(localtime())
As mentioned by Jonathan Leffler, the format has the shortcoming of not having timezone information.
POSIX 7 marked those functions as "obsolescent" so they could be removed in future versions:
The standard developers decided to mark the asctime() and asctime_r() functions obsolescent even though asctime() is in the ISO C standard due to the possibility of buffer overflow. The ISO C standard also provides the strftime() function which can be used to avoid these problems.
C++ version of this question: How to get current time and date in C++?
Tested in Ubuntu 16.04.
time_t rawtime;
time ( &rawtime );
struct tm *timeinfo = localtime ( &rawtime );
You can also use strftime to format the time into a string.
To expand on the answer by Ori Osherov
You can use the WinAPI to get the date and time, this method is specific to Windows, but if you are targeting Windows only, or are already using the WinAPI then this is definitly a possibility1:
You can get both the time and date by using the SYSTEMTIME struct. You also need to call one of two functions (either GetLocalTime() or GetSystemTime()) to fill out the struct.
GetLocalTime() will give you the time and date specific to your time zone.
GetSystemTime() will give you the time and date in UTC.
The SYSTEMTIME struct has the following members:
wYear, wMonth, wDayOfWeek, wDay, wHour, wMinute, wSecond and wMilliseconds
You then need to just access the struct in the regular way
Actual example code:
#include <windows.h> // use to define SYSTEMTIME , GetLocalTime() and GetSystemTime()
#include <stdio.h> // For printf() (could otherwise use WinAPI equivalent)
int main(void) { // Or any other WinAPI entry point (e.g. WinMain/wmain)
SYSTEMTIME t; // Declare SYSTEMTIME struct
GetLocalTime(&t); // Fill out the struct so that it can be used
// Use GetSystemTime(&t) to get UTC time
printf("Year: %d, Month: %d, Day: %d, Hour: %d, Minute:%d, Second: %d, Millisecond: %d", t.wYear, t.wMonth, t.wDay, t.wHour, t.wMinute, t.wSecond, t.wMilliseconds); // Return year, month, day, hour, minute, second and millisecond in that order
return 0;
}
(Coded for simplicity and clarity, see the original answer for a better formatted method)
The output will be something like this:
Year: 2018, Month: 11, Day: 24, Hour: 12, Minute:28, Second: 1, Millisecond: 572
Useful References:
All the WinAPI documentation (most already listed above):
GetLocalTime()
GetSystemTime()
SYSTEMTIME
Time Functions
An extremely good beginners tutorial on this subject by ZetCode:
https://zetcode.com/gui/winapi/datetime/
Simple operations with datetime on Codeproject:
https://www.codeproject.com/Articles/5546/WinAPI-Simple-Operations-with-datetime
1: As mentioned in the comments in Ori Osherov's answer ("Given that OP started with date +%F, they're almost certainly not using Windows. – melpomene Sep 9 at 22:17") the OP is not using Windows, however since this question has no platform specific tag (nor does it mention anywhere that the answer should be for that particular system), and is one of the top results when Googling "get time in c" both answers belong here, some users searching for an answer to this question may be on Windows and therefore will be useful to them.
Timespec has day of year built in.
http://pubs.opengroup.org/onlinepubs/7908799/xsh/time.h.html
#include <time.h>
int get_day_of_year(){
time_t t = time(NULL);
struct tm tm = *localtime(&t);
return tm.tm_yday;
}`
The answers given above are good CRT answers, but if you want you can also use the Win32 solution to this. It's almost identical but IMO if you're programming for Windows you might as well just use its API (although I don't know if you are programming in Windows).
char* arrDayNames[7] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
SYSTEMTIME st;
GetLocalTime(&st); // Alternatively use GetSystemTime for the UTC version of the time
printf("The current date and time are: %d/%d/%d %d:%d:%d:%d", st.wDay, st.wMonth, st.wYear, st.wHour, st.wMinute, st.wSecond, st.wMilliseconds);
printf("The day is: %s", arrDayNames[st.wDayOfWeek]);
Anyway, this is a Windows solution. I hope it will be helpful for you sometime!
I was using command line C-compiler to compile these and it completely drove me bonkers as it refused to compile.
For some reason my compiler hated that I was declaring and using the function all in one line.
struct tm tm = *localtime(&t);
test.c
test.c(494) : error C2143: syntax error : missing ';' before 'type'
Compiler Status: 512
First declare your variable and then call the function. This is how I did it.
char todayDateStr[100];
time_t rawtime;
struct tm *timeinfo;
time ( &rawtime );
timeinfo = localtime ( &rawtime );
strftime(todayDateStr, strlen("DD-MMM-YYYY HH:MM")+1,"%d-%b-%Y %H:%M",timeinfo);
printf("todayDateStr = %s ... \n", todayDateStr );
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
struct date
{
int month;
int day;
int year;
};
int calcN(struct date d)
{
int N;
int f(struct date d);
int g(int m);
N = 1461 * f(d) / 4 + 153 * g(d.month) / 5 + d.day;
if(d.year < 1700 || (d.year == 1700 && d.month < 3))
{
printf("Date must be after February 29th, 1700\n");
return 0;
}
else if(d.year < 1800 || (d.year == 1800 && d.month < 3))
N += 2;
else if(d.year < 1900 || (d.year == 1900 && d.month < 3))
N += 1;
return N;
}
int f(struct date d)
{
if(d.month <= 2)
d.year -= 1;
return d.year;
}
int g(int m)
{
if(m <=2)
m += 13;
else
m += 1;
return m;
}
int main(void)
{
int calcN(struct date d);
struct date d1, d2;
int N1, N2;
time_t t;
time(&t);
struct tm *now = localtime(&t);
d1.month = now->tm_mon + 1;
d1.day = now->tm_mday;
d1.year = now->tm_year + 1900;
printf("Today's date: %02i/%02i/%i\n", d1.month, d1.day, d1.year);
N1 = calcN(d1);
printf("Enter birthday (mm dd yyyy): ");
scanf("%i%i%i", &d2.month, &d2.day, &d2.year);
N2 = calcN(d2);
if(N2 == 0)
return 0;
printf("Number of days since birthday: %i\n", N1 - N2);
return 0;
}
#include <stdio.h>
int main() {
char *pts; /* pointer to time string */
time_t now; /* current time */
char *ctime();
(void) time(&now);
printf("%s", ctime(&now));
return(0);
}
Sample output:
Sat May 14 19:24:54 2022
This is the easiest way. I haven't even used time.h.
Be advised: The output produced has a newline at the end.
instead of files use pipes and if u wana use C and not C++ u can use popen like this
#include<stdlib.h>
#include<stdio.h>
FILE *fp= popen("date +F","r");
and use *fp as a normal file pointer with fgets and all
if u wana use c++ strings, fork a child, invoke the command and then pipe it to the parent.
#include <stdlib.h>
#include <iostream>
#include <string>
using namespace std;
string currentday;
int dependPipe[2];
pipe(dependPipe);// make the pipe
if(fork()){//parent
dup2(dependPipe[0],0);//convert parent's std input to pipe's output
close(dependPipe[1]);
getline(cin,currentday);
} else {//child
dup2(dependPipe[1],1);//convert child's std output to pipe's input
close(dependPipe[0]);
system("date +%F");
}
// make a similar 1 for date +T but really i recommend u stick with stuff in time.h GL
#include<stdio.h>
using namespace std;
int main()
{
printf("%s",__DATE__);
printf("%s",__TIME__);
return 0;
}