gmtoff

gmtoff is shorthand for the GMT offset used to convert between Coordinated Universal Time (UTC) and local time. In source code, gmtoff is most commonly realized as the tm_gmtoff member of the struct tm type on many Unix-like systems. This field holds the offset from UTC in seconds; positive values indicate time zones east of UTC, negative values indicate time zones west of UTC. Alongside tm_zone, which may hold a human-readable time zone abbreviation, tm_gmtoff provides a machine-friendly representation of the local offset.

The gmtoff field is not part of the C standard. Its presence and exact semantics can vary by platform and library implementation. In glibc and many BSD variants, struct tm includes both tm_gmtoff and tm_zone, and localtime and related functions populate tm_gmtoff with the current offset (including any DST adjustment) for the corresponding local time. However, portable code should not rely on tm_gmtoff being available or on its DST behavior; it is a common extension rather than a guaranteed standard feature.

DST considerations: The offset stored in gmtoff reflects the total offset from UTC, and the tm_isdst field indicates whether DST is in effect for that time. The interaction between gmtoff and DST can differ by implementation, so developers should consult their platform’s time API documentation when precise behavior is required.

Portability: Windows and some non-Unix environments may not provide tm_gmtoff. In portable code, use standard time APIs and, when necessary, compute or retrieve offsets via supported facilities and format time according to ISO 8601 or locale-specific rules.