gmtime, gmtime_r, gmtime_s

Defined in header `<time.h>`
struct tm* gmtime ( const time_t* timer );	(1)
struct tm* gmtime_r( const time_t* timer, struct tm* buf );	(2)	(since C23)
struct tm* gmtime_s( const time_t* restrict timer, struct tm* restrict buf );	(3)	(since C11)

1) Converts given time since epoch (a time_t value pointed to by timer) into calendar time, expressed in Coordinated Universal Time (UTC) in the struct tm format. The result is stored in static storage and a pointer to that static storage is returned.

2) Same as (1), except that the function uses user-provided storage buf for the result.

3) Same as (1), except that the function uses user-provided storage buf for the result and that the following errors are detected at runtime and call the currently installed constraint handler function:

timer or buf is a null pointer

As with all bounds-checked functions, gmtime_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <time.h>.

Parameters

timer	-	pointer to a time_t object to convert
buf	-	pointer to a struct tm object to store the result

Return value

1) pointer to a static internal tm object on success, or null pointer otherwise. The structure may be shared between gmtime, localtime, and ctime and may be overwritten on each invocation.

2,3) copy of the buf pointer, or null pointer on error (which may be a runtime constraint violation or a failure to convert the specified time to UTC).

Notes

gmtime may not be thread-safe.

POSIX requires that gmtime and gmtime_r set errno to EOVERFLOW if they fail because the argument is too large.

The implementation of gmtime_s in Microsoft CRT is incompatible with the C standard since it has reversed parameter order.

Example

Run this code

#define __STDC_WANT_LIB_EXT1__ 1
#define _XOPEN_SOURCE // for putenv
#include <stdio.h>
#include <stdlib.h>   // for putenv
#include <time.h>
 
int main(void)
{
    time_t t = time(NULL);
    printf("UTC:       %s", asctime(gmtime(&t)));
    printf("local:     %s", asctime(localtime(&t)));
    // POSIX-specific
    putenv("TZ=Asia/Singapore");
    printf("Singapore: %s", asctime(localtime(&t)));
 
#ifdef __STDC_LIB_EXT1__
    struct tm buf;
    char str[26];
    asctime_s(str, sizeof str, gmtime_s(&t, &buf));
    printf("UTC:       %s", str);
    asctime_s(str, sizeof str, localtime_s(&t, &buf));
    printf("local:     %s", str);
#endif
}

Possible output:

UTC:       Fri Sep 15 14:22:05 2017
local:     Fri Sep 15 14:22:05 2017
Singapore: Fri Sep 15 22:22:05 2017
UTC:       Fri Sep 15 14:22:05 2017
local:     Fri Sep 15 14:22:05 2017

References

C23 standard (ISO/IEC 9899:2024):

7.27.3.3 The gmtime function (p: TBD)

K.3.8.2.3 The gmtime_s function (p: TBD)

C17 standard (ISO/IEC 9899:2018):

7.27.3.3 The gmtime function (p: 288)

K.3.8.2.3 The gmtime_s function (p: 454-455)

C11 standard (ISO/IEC 9899:2011):

7.27.3.3 The gmtime function (p: 393-394)

K.3.8.2.3 The gmtime_s function (p: 626-627)

C99 standard (ISO/IEC 9899:1999):

7.23.3.3 The gmtime function (p: 343)

C89/C90 standard (ISO/IEC 9899:1990):

4.12.3.3 The gmtime function

Compiler support
Language
Headers
Type support
Program utilities
Variadic function support
Error handling
Dynamic memory management
Strings library
Algorithms
Numerics
Date and time utilities
Input/output support
Localization support
Concurrency support (C11)
Technical Specifications
Symbol index

Functions
Time manipulation
difftime
time
clock
timespec_get (C11)
timespec_getres (C23)
Format conversions
asctimeasctime_s (deprecated in C23)(C11)
ctimectime_s (deprecated in C23)(C11)
strftime
wcsftime (C95)
gmtimegmtime_rgmtime_s (C23)(C11)
localtimelocaltime_rlocaltime_s (C23)(C11)
mktime
Constants
CLOCKS_PER_SEC
Types
tm
time_t
clock_t
timespec (C11)