TZID

TZID is a parameter used in the iCalendar data standard (RFC 5545) to identify the time zone of a date-time value. It appears on local date-time properties such as DTSTART and DTEND to specify how the time should be interpreted with respect to time zone rules, including daylight saving time.

The TZID value is typically a string that denotes an IANA time zone identifier, such as America/New_York or Europe/London. It may also refer to a VTIMEZONE component defined within the same calendar file, which contains the actual time zone rules (standard time and daylight saving time) and their offsets over time. Using TZID allows calendar applications to render and compute local times consistently, especially for events that occur across DST transitions or in different regions.

In practice, the presence of TZID means the associated date-time is zone-specific rather than a simple offset. If a TZID cannot be resolved by a calendar client (for example, if the time zone database is missing or the value is unrecognized), interpretation of the time can become ambiguous. Some clients may fall back to a local system time zone, use UTC, or flag an error.

TZID is distinct from explicit UTC times (which use a trailing Z) or floating times (which have no TZID). While many ICS files use UTC or floating times for simplicity, TZID-enabled date-times provide precise, region-aware scheduling across different locales, making them essential for accurate event timings in international calendars.