localtime

Localtime is a function in the C standard library that converts a time value to a broken-down calendar time in the system’s local time zone. The input is a pointer to time_t, and the function returns a pointer to a statically allocated struct tm containing fields such as tm_year, tm_mon, tm_mday, tm_hour, tm_min, tm_sec, tm_wday, tm_yday, and tm_isdst. The resulting values reflect the local offset from UTC and any daylight saving time (DST) adjustments as determined by the system’s time zone data.

The function uses the system’s time zone settings, which may be influenced by environment configuration (for example, the TZ variable in many Unix-like systems) or platform-specific time zone databases. Local time can be ambiguous or invalid during DST transitions, and tm_isdst indicates DST status when available.

Because localtime returns a pointer to a static internal buffer, its contents can be overwritten by subsequent calls. This makes localtime non-thread-safe in many environments. To obtain thread-safe results, many systems provide localtime_r (POSIX) or localtime_s (Windows), which store the result in a user-supplied struct tm.

Related functions include gmtime, which converts time_t to Coordinated Universal Time (UTC) without regard to local time zone, and mktime, which converts a local broken-down time back to time_t using the local time rules. The concept of local time and its DST rules can vary by region and over time as time zone databases are updated.