BYSECOND

BYSECOND is a rule component of the iCalendar RRULE specification that designates which seconds within a minute an event should occur when evaluating recurring times. It is one of the BYxxx rule parts used to refine recurrence patterns, alongside BYMINUTE, BYHOUR and others. BYSECOND specifies the allowed seconds in each minute for occurrences, enabling sub-minute timing in calendars that support it.

Values for BYSECOND are integers in the range 0 to 60, with 60 occasionally used to denote a leap second in some implementations. The BYSECOND rule is typically applied in conjunction with other rule parts and a defined FREQ value (for example, FREQ=MINUTELY), so that the resulting instances occur at specific seconds within each minute. Because BYSECOND operates on the seconds within a minute, it commonly refines times produced by more coarse parts such as BYMINUTE or BYHOUR.

Examples illustrate the intended use: RRULE:FREQ=MINUTELY;BYSECOND=0,30 would generate an occurrence at second 0 and second 30 of every minute. RRULE:FREQ=MINUTELY;BYSECOND=15 would generate one event at the 15th second of every minute. In practice, exact behavior can depend on the calendar system and how it interprets combinations of rule parts, so implementation details may vary.

Compatibility and notes: BYSECOND is defined in RFC 5545 (iCalendar), but not all calendar clients fully support sub-minute rules. Some applications may ignore BYSECOND or require specific combinations of RRULE parts. When exchanging events, verify that participants’ calendars handle BYSECOND as intended.

See also: RRULE, BYMINUTE, BYHOUR, RFC 5545.