Functions
TZISDST
TRUE if daylight-saving is in effect at this instant
Try it
When to use it
When should I use TZISDST instead of comparing the offset to a fixed value?
Use TZISDST when you need a clear TRUE/FALSE flag for DST rather than inferring it from an offset. Some zones share the same UTC offset in both standard and summer time, so offset comparison alone is unreliable.
What does TZISDST return for a zone that never observes daylight saving time?
It returns FALSE for every instant in zones like UTC, Asia/Tokyo, or Africa/Nairobi because those zones have no DST transition rules.
How do I use TZISDST to adjust a financial calculation only during DST?
Wrap the calculation in an IF: use TZISDST to detect whether DST is active, then apply the DST-adjusted rate or schedule in the true branch and the standard one in the false branch.
Use cases
Check DST status for a London market open in winter
Conditionally label a timestamp as standard or summer time
Used in
finance · operations · engineering
Syntax
TZISDST(zoned)