tm

(3type)

broken-down time

#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 */ 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

LIBRARY

Standard C library (libc)

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.