isInDaylightSaving

Signature

isInDaylightSaving(value: string): boolean

Check whether a zoned value’s instant falls within daylight saving time.

• The rule is GMT’s own definition, not the tz database’s daylight flag. No JavaScript API exposes that flag, and it cannot be worked out from offsets.
• The rule: daylight time runs from a forward change of the zone’s clocks to the backward change that undoes it. A backward change undoes the most recent forward change of the same size not yet undone, if it comes less than 365 days after it. The instant is in daylight time when it is at or after such a forward change and before its backward change.
• Read from the zone’s offset changes in the runtime’s time zone data (getTimeZoneTransition); GMT holds no zone data. The answer changes only at a clock change, and the answer for a past instant can change when the runtime’s data learns that a zone stopped changing its clocks.
• A forward change never undone, or undone 365 days or more later, is a change of standard time: Europe/Istanbul is on standard time from its last advance in March 2016.
• The higher of two alternating offsets is the daylight one, as in the tz database’s rearguard form. Its main form reverses Europe/Dublin (winter is its daylight time, a negative save) and Africa/Casablanca (the Ramadan weeks are); here Dublin’s summer and Casablanca’s months at +01:00 are daylight time.
• Read as standard time, because the offsets do not show otherwise:
• the last summer before a zone kept its daylight offset for good (the tz database dates Istanbul’s switch 2016-09-07, with no clock change);
• a daylight period held 365 days or longer (America/Santiago 2014 to 2016);
• a daylight period that began, ended or was interrupted by a move of standard time, so the clocks went back by another amount or not at all (America/Kentucky/Monticello 2000), or the offset changed part-way through and the rest, or all, of the period is not paired (Asia/Tomsk 2002, Asia/Jerusalem 1948);
• a daylight period whose end is past the last instant Temporal can represent.
• Read as daylight time: a move of standard time reversed by the same amount less than 365 days later (America/Metlakatla 2018 to 2019).
• The other DST functions answer other questions: hasDaylightSaving(timeZone, { at }) whether a zone observes DST at an instant, getDstTransitions(timeZone, year) where its offset changes fall, and classifyLocal whether a wall time is ambiguous. See docs/dst-disambiguation.md.
• Returns false for invalid input, and for a zone with a fixed offset.

Parameters

Parameter	Type	Description
value	string	zoned ISO 8601 datetime string

Returns

true if the instant is in daylight saving time, false otherwise or on invalid input

Examples

import { isInDaylightSaving } from "@northguild/gmt/zoned/compare";

isInDaylightSaving("2024-07-15T12:00:00-04:00[America/New_York]") // true

value

output

 

Source

zoned/compare/isInDaylightSaving.ts