isdst

isdst is a shorthand commonly used in computing to refer to daylight saving time status. In practice, it most often denotes a flag that indicates whether daylight saving time is in effect for a given moment, and is closely associated with the tm_isdst field of the standard C struct tm.

In the C programming interface, struct tm contains an integer field named tm_isdst. The value indicates the DST status: typically 1 if daylight saving time is in effect, 0 if it is not, and -1 when the information is unavailable or not applicable. This flag is used by time conversion functions such as localtime and mktime to apply the correct time offset according to the local time zone rules.

The actual DST rules are defined by the system’s time zone database, such as the IANA Time Zone Database. Because DST rules can change and vary by region, the tm_isdst value may differ between platforms or contexts, and some times may be ambiguous during DST transition moments.

Limitations and considerations include the fact that tm_isdst may be set to -1 to indicate that the DST status could not be determined, and relying solely on the flag can be unsafe if time zone data is outdated or incomplete. For portable or robust time handling, developers often rely on higher-level time zone libraries or system facilities that manage DST transitions and updates.

Name variations exist across languages and libraries; while isdst is a common shorthand, the corresponding field or indicator is typically implemented as tm_isdst within the struct tm. See also daylight saving time, time zone databases, localtime, mktime, and struct tm.