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
DST considerations: The offset stored in gmtoff reflects the total offset from UTC, and the tm_isdst field
Portability: Windows and some non-Unix environments may not provide tm_gmtoff. In portable code, use standard time