How to convert from UTC to local time in C?

Question

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(&lt);

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.

Answer 1

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.

Answer 2

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.

Answer 3

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;
}

Answer 4

//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;}
        }
    }

Answer 5

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

Answer 6

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;
}

Answer 7

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).

Answer 8

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;
}

Answer 9

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 (&current_time_MST);        // MST

Enjoy.

Answer 10

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);   
}