tm(3type)

NAME
tm - broken-down time

LIBRARY
Standard C library (libc)

SYNOPSIS
    #include <time.h>

struct tm {
    int         tm_sec;    /* Seconds          [0, 60] */
    int         tm_min;    /* Minutes          [0, 59] */
    int         tm_hour;   /* Hour             [0, 23] */
    int         tm_mday;   /* Day of the month [1, 31] */
    int         tm_mon;    /* Month            [0, 11]  (January = 0) */
    int         tm_year;   /* Year minus 1900 */
    int         tm_wday;   /* Day of the week  [0, 6]   (Sunday = 0) */
    int         tm_yday;   /* Day of the year  [0, 365] (Jan/01 = 0) */
    int         tm_isdst;  /* Daylight savings flag */

    /* Since POSIX.1-2024:  */
    long        tm_gmtoff; /* Seconds East of UTC */
    const char *tm_zone;   /* Timezone abbreviation */
};

    Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

tm_gmtoff, tm_zone:
    Since glibc 2.20:
        _DEFAULT_SOURCE
    glibc 2.20 and earlier:
        _BSD_SOURCE

DESCRIPTION
Describes time, broken down into distinct components.

tm_isdst describes whether daylight saving time is in effect at the time described. The value is positive if daylight saving time is in effect, zero if it is not, and negative if the information is not available.

tm_gmtoff is the difference, in seconds, of the timezone represented by this broken-down time and UTC (this is the additive inverse of timezone(3)).

tm_zone is the equivalent of tzname(3) for the timezone represented by this broken-down time.

VERSIONS
In C90, tm_sec could represent values in the range [0, 61], which could represent a double leap second. UTC doesn't permit double leap seconds, so it was limited to 60 in C99.

timezone(3), as a variable, is an XSI extension: some systems provide the V7-compatible timezone(3) function. The tm_gmtoff field provides an alternative (with the opposite sign) for those systems.

tm_zone points to static storage and may be overridden on subsequent calls to localtime(3) and similar functions (however, this never happens under glibc).

STANDARDS
C23, POSIX.1-2024.

HISTORY
C89, POSIX.1-1988.

tm_gmtoff and tm_zone originate from 4.3BSD-Tahoe (where tm_zone is a char *), and were first standardized in POSIX.1-2024.

NOTES
tm_sec can represent a leap second with the value 60.

SEE ALSO
ctime(3), strftime(3), strptime(3), time(7)