Overview #

The gmtime() function converts a calendar time stored as a time_t value into a broken-down UTC (Coordinated Universal Time).

Like localtime(), it returns a pointer to a struct tm containing individual date and time components. However, the returned time is expressed in UTC, not the local time zone.

Header File #

#include <time.h>

Man Pages #

man 3 gmtime

Function Prototype #

struct tm *gmtime(const time_t *timer);

Parameters #

Return Value #

On success, gmtime() returns a pointer to a struct tm containing the UTC date and time.

On failure, it returns:

NULL

Data Types #

struct tm #

struct tm stores the individual components of a date and time.

struct tm
{
    int tm_sec;
    int tm_min;
    int tm_hour;
    int tm_mday;
    int tm_mon;
    int tm_year;
    int tm_wday;
    int tm_yday;
    int tm_isdst;
};

Examples #

Example 1 – Display UTC Date and Time #

main.c #

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

int main(void)
{
    time_t current_time;
    struct tm *utc;

    current_time = time(NULL);

    utc = gmtime(&current_time);

    printf("Date : %02d-%02d-%04d\n",
           utc->tm_mday,
           utc->tm_mon + 1,
           utc->tm_year + 1900);

    printf("Time : %02d:%02d:%02d UTC\n",
           utc->tm_hour,
           utc->tm_min,
           utc->tm_sec);

    return 0;
}

Output #

Date : 27-07-2026
Time : 10:15:32 UTC

The output will be different on your system.

Explanation #

current_time = time(NULL);

Obtains the current calendar time.

utc = gmtime(&current_time);

Converts the calendar time into a struct tm representing the UTC date and time.

utc->tm_mon + 1

tm_mon stores months in the range 0–11, where January is 0. Adding 1 converts it to the familiar range 1–12.

utc->tm_year + 1900

tm_year stores the number of years since 1900. Adding 1900 gives the actual year.

Example 2 – Compare Local Time and UTC #

main.c #

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

int main(void)
{
    time_t current_time;
    struct tm *local;
    struct tm *utc;

    current_time = time(NULL);

    local = localtime(&current_time);
    utc = gmtime(&current_time);

    printf("Local Time : %02d:%02d:%02d\n",
           local->tm_hour,
           local->tm_min,
           local->tm_sec);

    printf("UTC Time   : %02d:%02d:%02d\n",
           utc->tm_hour,
           utc->tm_min,
           utc->tm_sec);

    return 0;
}

Output #

Local Time : 15:45:32
UTC Time   : 10:15:32

The output will vary depending on your local time zone.

Explanation #

local = localtime(&current_time);

Converts the calendar time into the local date and time.

utc = gmtime(&current_time);

Converts the same calendar time into UTC.

The difference between the two values corresponds to your system’s configured time zone.

Notes #

• gmtime() converts a time_t value into UTC (Coordinated Universal Time).
• It returns a pointer to a struct tm.
• tm_mon ranges from 0 to 11, so add 1 when displaying the month.
• tm_year represents the number of years since 1900, so add 1900 to obtain the actual year.
• The returned pointer refers to statically allocated memory managed by the C library.
• Each call to gmtime() may overwrite the result of a previous call.
• Do not modify or free the returned structure.

Related Functions #

• time()
• localtime()
• ctime()
• mktime()
• strftime()