`gmtime_s`, `_gmtime32_s`, `_gmtime64_s`

2025-10-25

Converts a time value to a tm structure. These functions are versions of _gmtime32, _gmtime64 with security enhancements as described in Security features in the CRT.

Syntax

errno_t gmtime_s( // See note in remarks section about linkage
   struct tm* tmDest,
   const __time_t* sourceTime
);
errno_t _gmtime32_s(
   struct tm* tmDest,
   const __time32_t* sourceTime
);
errno_t _gmtime64_s(
   struct tm* tmDest,
   const __time64_t* sourceTime
);

Parameters

tmDest
Pointer to a tm structure. The fields of the returned structure hold the evaluated value of the timer argument in UTC rather than in local time.

sourceTime
Pointer to stored time. The time is represented as seconds elapsed since midnight (00:00:00), January 1, 1970, coordinated universal time (UTC).

Return value

Zero if successful. The return value is an error code if there's a failure. Error codes are defined in Errno.h; for a listing of these errors, see errno.

Error conditions

`tmDest`	`sourceTime`	Return	Value in `tmDest`
`NULL`	any	`EINVAL`	Not modified.
Not `NULL` (points to valid memory)	`NULL`	`EINVAL`	All fields set to -1.
Not `NULL`	< 0	`EINVAL`	All fields set to -1.

The first two error conditions invoke the invalid parameter handler, as described in Parameter validation. If execution is allowed to continue, these functions set errno to EINVAL and return EINVAL.

Remarks

The _gmtime32_s function breaks down the sourceTime value and stores it in a structure of type tm, defined in Time.h. The address of the structure is passed in tmDest. The value of sourceTime is often obtained from a call to the time function.

Each of the structure fields is of type int, as shown in the following table.

Field	Description
`tm_sec`	Seconds after minute (0 - 59).
`tm_min`	Minutes after hour (0 - 59).
`tm_hour`	Hours since midnight (0 - 23).
`tm_mday`	Day of month (1 - 31).
`tm_mon`	Month (0 - 11; January = 0).
`tm_year`	Year (current year minus 1900).
`tm_wday`	Day of week (0 - 6; Sunday = 0).
`tm_yday`	Day of year (0 - 365; January 1 = 0).
`tm_isdst`	Always 0 for `gmtime_s`.

_gmtime64_s, which uses the __time64_t structure, allows dates to be expressed up through 23:59:59, December 31, 3000, UTC; whereas gmtime32_s only represent dates through 23:59:59 January 18, 2038, UTC. Midnight, January 1, 1970, is the lower bound of the date range for both these functions.

gmtime_s is an inline function that evaluates to _gmtime64_s and time_t is equivalent to __time64_t. If you need to force the compiler to interpret time_t as the old 32-bit time_t, you can define _USE_32BIT_TIME_T. _USE_32BIT_TIME_T causes gmtime_s to be inlined as _gmtime32_s. We don't recommend _USE_32BIT_TIME_T, because your application may fail after January 18, 2038, and because it isn't allowed on 64-bit platforms.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Note

When you use Windows SDK version 10.0.26100.6901 and Visual Studio 2026 or later together, gmtime_s is no longer static inline (internal linkage). Instead, it's inline (external linkage).
To return to the previous behavior, #define _STATIC_INLINE_UCRT_FUNCTIONS=1 before including any CRT headers. By default, _STATIC_INLINE_UCRT_FUNCTIONS is set to 0.
This change increases UCRT conformance with the C++ standard and improves compatibility with C++ modules.

Requirements

Routine	Required C header	Required C++ header
`gmtime_s`, `_gmtime32_s`, `_gmtime64_s`	`<time.h>`	`<ctime>` or `<time.h>`

For more compatibility information, see Compatibility.

Example

// crt_gmtime64_s.c
// This program uses _gmtime64_s to convert a 64-bit
// integer representation of coordinated universal time
// to a structure named newtime, then uses asctime_s to
// convert this structure to an output string.

#include <time.h>
#include <stdio.h>

int main( void )
{
   struct tm newtime;
   __int64 ltime;
   char buf[26];
   errno_t err;

   _time64( &ltime );

   // Obtain coordinated universal time:
   err = _gmtime64_s( &newtime, &ltime );
   if (err)
   {
      printf("Invalid Argument to _gmtime64_s.");
   }

   // Convert to an ASCII representation
   err = asctime_s(buf, 26, &newtime);
   if (err)
   {
      printf("Invalid Argument to asctime_s.");
   }

   printf( "Coordinated universal time is %s\n",
           buf );
}

Coordinated universal time is Fri Apr 25 20:12:33 2003

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