C programming - calculate differences of 2 date times - c

Question

I am trying to use getDateTime to calculate the difference between two different time dates: It suppose to be something like this:
Enter date and time (yyyy-mm-dd hh:mm:ss): 2017-03-13 12:00:00 Start: Mon Mar 13 12:00:00 2017
Enter date and time (yyyy-mm-dd hh:mm:ss): 2017-03-13 12:00:30 End: Mon Mar 13 12:00:30 2017
Difference: 30 seconds.
This is my attempt:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int dayOfWeek(int d, int m, int y){
// Compute day of week: 0 Sun 1 Mon...6 Sat
int z;
if(m<3){
z=y;
y=y-1;
}
else z=y-2;
return (23*m/9 + d + z + 4 +
y/4- y/100 + y/400)%7;
}
int getDateTime(struct tm *t){
int year,month,day,hrs,min,sec;
printf("Enter date and time (yyyy-mm-dd hh:mm:ss): ");
scanf("%d-%d-%d", &year, &month, &day);
scanf("%d:%d:%d",&hrs,&min,&sec);
int x;
t->tm_year = year - 1900;
t->tm_mon = month - 1;
t->tm_mday = day;
t->tm_hour = hrs;
t->tm_min = min;
t->tm_sec = sec;
t->tm_wday = dayOfWeek(day,month,year);
return ;
}
int main()
{
setvbuf(stdout, NULL, _IONBF, 0);
struct tm Time1 , Time2;
double x;
getDateTime(&Time1);
printf("Start: %s", asctime(&Time1));
getDateTime(&Time2);
printf("End: %s", asctime(&Time2));
return EXIT_SUCCESS;
}
I was able to print it in the correct format, but how do I calculate the difference in seconds ? I tried to use difftime but I think I use it wrong. Many thanks !

Answer 1

There's already a function to parse dates, strptime. It's like scanf for dates. It follows the same formatting codes as strftime (p for parse, f for format).
You can do it long hand.
struct tm date;
strptime( "2017-03-13 12:00:00", "%Y-%m-%s %H:%M:%S", &date );
Or use the short hands provided for ISO 8601 dates and times.
struct tm date;
strptime( "2017-03-13 12:00:00", "%F %T", &date );
That replaces getDateTime.
Once that's done and you have the dates as struct tm, you can convert them to seconds since the epoch (1970-01-01 00:00:00 UTC) and subtract. Converting to seconds since the epoch, time_t, is done using mktime. Then subtract with difftime.
time_t seconds1 = mktime(&date1);
time_t seconds2 = mktime(&date2);
double timediff = difftime( seconds1, seconds2 );
Though, quite honestly, you can also just seconds1 - seconds2. difftime was added to the standard to allow time_t to not be a simple number, but I know of no instance of that.
mktime assumes the date is in the local time zone. That doesn't matter for the purposes of diffing two dates in the same time zone.
If it does, there's the non-standard function timegm, which is mktime but it assumes they're UTC dates. Or you can write your own using the POSIX functions setenv and tzset to change the time zone temporarily.
time_t my_timegm(struct tm *date) {
// Store the current TZ value.
char *tz = getenv("TZ");
// Unset TZ which will cause UTC to be used.
setenv("TZ", "", 1);
tzset();
// Call mktime, now in UTC.
time_t time = mktime(date);
// Restore the TZ environment variable (or leave it unset).
if (tz)
setenv("TZ", tz, 1);
else
unsetenv("TZ");
tzset();
return time;
}