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I'm writing a function that, among other things, must print out an elapsed time counter. It receives by reference a start time _start, and compares it to current, both typed as time_t. I want to use strftime() to print out the observed time delta in ISO 8601 format. Here's what I attempted to do:
// Negative start time implies program has not begun
if (*_start > 0) {
time_t elapsed = current - *_start;
strftime(time_str, sizeof(time_str) - 1, "%T", localtime(&elapsed));
printf("%s\n", time_str);
}
And here is the output I get immediately after running the program
01:00:00
01:00:00
01:00:01
01:00:01
01:00:01
The seconds work fine, if I let it run longer they get incremented as expected, so do the minutes, however the hour starts as 01 as opposed to 00. Why is this happening? How can I get the hours to start zeroed, like the minutes?
time_t typically (see edit) stores absolute timestamps (the number of seconds since midnight UTC, January 1, 1970). By calculating current - *_start, you're getting elapsed time in seconds (as desired), but by then passing that to localtime and strftime, you're telling the computer to take the time elapsed since the start of your program and treat it as the time elapsed since midnight UTC 1-1-1970.
I'm guessing that happens to be 01:00:00 in your system's local time zone.
I'm not aware of a C99 function to print elapsed time, but it's not hard to write one yourself.
void format_elapsed_time(char *time_str, int total_seconds_elapsed) {
int hours_elapsed = total_seconds_elapsed / 3600;
int minutes_elapsed = (total_seconds_elapsed % 3600) / 60;
int seconds_elapsed = total_seconds_elapsed % 60;
sprintf(time_str, "%02i:%02i:%02i", hours_elapsed, minutes_elapsed, seconds_elapsed);
}
Edit: As #chux points out, time_t doesn't have to store timestamps as seconds since 1-1-1970. See this answer for details.
To portably find the number of elapsed seconds between 2 time_t, use difftime() as subtracting 2 time_t values is not specified in C to be a difference in seconds.
double difftime(time_t time1, time_t time0); C11dr §7.26.2.2
The difftime function returns the difference expressed in seconds as a double.
double elapsed_seconds = difftime(current, *_start);
printf("elapsed_seconds: %f\n", elapsed_seconds);
If you want to use this method, you should be aware that time() returns UTC. If you then subtract that from your local time (as returned by localtime()), time zones will have an effect on the result (it's likely your particular time zone is removed from UTC by one hour).
Consider the following complete program, similar to your snippet:
#include <stdio.h>
#include <time.h>
#include <unistd.h>
int main(void) {
time_t start = time(0);
char time_str[100];
for (int i = 5; i > 0; i--) {
sleep (1);
time_t elapsed = time(0) - start;
strftime(time_str, sizeof(time_str) - 1, "%T", localtime(&elapsed));
printf("%s\n", time_str);
}
return 0;
}
Also consider that my particular time zone is removed from UTC by eight hours:
pax> date ; date -u
Thursday 30 March 10:25:57 AWST 2017
Thursday 30 March 02:25:57 UTC 2017
When I run it, I see:
08:00:01
08:00:02
08:00:03
08:00:04
08:00:05
You can see there that the eight-hour time difference from UTC affects the value. The fix for that is actually quite simple: don't use local time at all. If you replace the localtime() call with gmtime(), you get the output you expect:
00:00:01
00:00:02
00:00:03
00:00:04
00:00:05
I realize this question already has an answer, but I thought I might shift the focus a little and expand on the accepted answer. Since the OP specified that they are dealing with elapsed times, one can avoid dealing with absolute timestamps altogether using the clock() function.
clock_t start = clock();
.
.
clock_t end = clock();
double elapsed = (end - start) / (double)CLOCKS_PER_SEC;
then you can call the notionally revised format_elapsed_time() function, like so:
void format_elapsed_time(char *time_str, double elapsed) {
int h, m, s, ms;
h = m = s = ms = 0;
ms = elapsed * 1000; // promote the fractional part to milliseconds
h = ms / 3600000;
ms -= (h * 3600000);
m = ms / 60000;
ms -= (m * 60000);
s = ms / 1000;
ms -= (s * 1000);
sprintf(time_str, "%02i:%02i:%02i.%03i", h, m, s, ms);
}
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.
Is there any way of converting milliseconds to date in C?
What I am trying to do is write a small application in C that can return the financial year and the like(quarter, week) given the start month and isCurentYear bool, where the input might be milliseconds or a date!
In the first place, is there any way by which this can be achieved in C?
And if so, in the process of finding out a way of converting milliseconds to date
I have found out that the use of time_t takes the current millis of our system and by creating a structure pointing to it,it permits us to extract the year,month, date, sec etc!
Refer the below code:
#include <sys/time.h>
#include<stdio.h>
#include<time.h>
void main()
{
time_t t = time(000);
//time_t t = time(0);
struct tm tm = *localtime(&t);
printf("now: %d-%d-%d %d:%d:%d\n", tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
tm.tm_hour, tm.tm_min, tm.tm_sec);
}
And also, can time_t be used to store millisecond values so that it can be converted to date using tm struct?
Function time_t time(time_t* timer) returns the number of seconds elapsed since 00:00 hours, Jan 1, 1970 UTC. In addition, if the input argument timer != NULL, then the function also sets this argument to the same value (so you probably have no reason to call it with anything else but NULL).
Function struct tm* localtime(const time_t* timer) takes the number of seconds elapsed since 00:00 hours, Jan 1, 1970 UTC, and returns a structure that represents the equivalent time & date. If you're working on a multi-threaded application, then please note that this function is not thread safe.
As to your question - is there any way for converting milliseconds to time & date - yes, but:
Take into consideration that the milliseconds will be considered as of 00:00 hours, Jan 1, 1970 UTC.
Since the time_t type is 32-bit long, you will not be able to convert 4G*1000 milliseconds or more.
Here is a function for converting milliseconds to time & date:
struct tm* GetTimeAndDate(unsigned long long milliseconds)
{
time_t seconds = (time_t)(milliseconds/1000);
if ((unsigned long long)seconds*1000 == milliseconds)
return localtime(&seconds);
return NULL; // milliseconds >= 4G*1000
}
For those of us who were searching the web for an answer to apply to embedded c applications, think pic 32 programming here is the mathematical calculation:
Date in Epoch_seconds = ( (epoch_seconds / 1000) / 86400 ) + 25569
Resulting in a 5 digit answer which is 10 bits long format dd/MM/yyyy
(Note: the slashes are encoded in the result here so when converting to human readable date please account for it)
Where one day = 86400 ms
and the date 1970/1/1 = 25569
example:=( (1510827144853/1000) / 86400 ) + 25569 = 43055
put 43055 in excel and format cell to date dd/MM/yyyy and it gives you 16/11/2017
Perhaps, you are looking for strftime function.
char text[100];
time_t now = time(NULL);
struct tm *t = localtime(&now);
strftime(text, sizeof(text)-1, "%d %m %Y %H:%M", t);
printf("Current Date: %s", text);
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.)
I'm trying to get the UTC time from a localtime. Vice versa it works OK. I only have problems when I want to convert a localtime to a UTC one in order to update an RTC clock. This is what I'm doing:
Say that t is a local timestamp.
char *tz = "GMT-2GMT,M3.5.0/3,M10.5.0/4";
t = 1311444000; // 23/07/11 18:00:00
set_TZ(tz);
gmt_time = gmtime(&t);
mktime(gmt_time);
printf("GMT Time: %s\r\n",asctime (gmt_time));
This gives me 18:00:00, when it should be minus the timezone.
How do I fix this problem?
The problem here is that time_t is supposed to represent the number of seconds (simplification, see footnote) since the epoch. This is not affected by time zones. If you add a time zone offset to a time_t, none of the functions will work as expected. Part of the problem is that it is sometimes impossible to know how to correctly convert such a value to a POSIX timestamp -- when the clocks roll back in the fall, where I live the clocks will read 1:30 AM twice during the same day, and without more information you can't figure out what the UTC time is.
You'll need to convert to local calendar time before you can convert to a POSIX timestamp, at which point you can convert to UTC calendar time.
// NOTE: not reentrant
time_t local_to_posix(time_t t) {
struct tm *tm;
time_t tt;
tm = gmtime(&t); // No timezone compensation
tm->tm_isdst = -1; // Let mktime figure out daylight savings
// NOTE: this WILL be wrong for one hour each year
tt = mktime(tm); // Converts local time to POSIX timestamp
return tt;
}
You can then pass the result to gmtime, which will give you the UTC calendar time.
Footnote: Strictly speaking, these timestamps are not UTC and do not measure the number of seconds since epoch. They are POSIX timestamps, which count seconds since epoch as if there were no leap seconds. This is probably irrelevant unless your realtime clock is an atomic clock.
After further tests the following code this seems to work. Im in UTC+2 and currently in DST. The returned time for UTC is correct. I will to further test without DST and see what is returned.
struct tm tm;
struct tm *local_time;
char *tz = "GMT-2GMT,M3.5.0/3,M10.5.0/4";
time_t t;
tm.tm_hour = 18;
tm.tm_min = 0;
tm.tm_sec = 0;
tm.tm_wday = 0;
tm.tm_yday = 0;
tm.tm_year = (2011) - 1900;
tm.tm_mday = 22;
tm.tm_mon = 7 - 1;
tm.tm_isdst = -1;
set_TZ(tz);
t = mktime(&tm);
local_time = localtime(&t);
printf("Local Time: %s\r\n",asctime (local_time ));
local_time = gmtime(&t);
printf("UTC Time: %s\r\n",asctime (local_time ));
Output:
Local Time: Fri Jul 22 18:00:00 2011
UTC Time: Fri Jul 22 15:00:00 2011