hasDaylightSaving

Signature

hasDaylightSaving(timeZone: string, options?: { at?: string; }): boolean

Check whether a timeZone observes daylight saving time at a reference instant.

• True when the zone is in daylight time at the reference instant, or a daylight period begins less than 365 days after it. So a zone in its winter is true, and a zone that has stopped changing its clocks is false.
• With options.at the answer does not depend on the day the code runs.
• The rule is GMT’s own definition, not the tz database’s daylight flag, which no JavaScript API exposes: daylight time runs from a forward change of the zone’s clocks to the backward change of the same size that undoes it, less than 365 days later.
• Read as standard time, because the offsets do not show otherwise: the last summer before a zone kept its daylight offset for good; a daylight period held 365 days or longer; one that began, ended or was interrupted by a move of standard time, so the offset changes part-way through and the rest, or all, of it is not paired (Asia/Tomsk 2002); and one whose end is past the last instant Temporal can represent.
• A forward change never undone is a change of standard time: Europe/Istanbul is false from 2016 on, although its clocks went forward in March 2016.
• Read from the runtime’s time zone data, so the answer can change when that data does.
• Returns false for an invalid timeZone, an invalid at, an options argument that is not an object, and a fixed UTC offset, with or without seconds ("+05:30", "-00:44:30").

Parameters

Parameter	Type	Description
timeZone	string	IANA name or UTC offset: a time zone identifier (+05:30, +0530, -08) or a stored offset (±HH:MM[:SS], what getTimeZoneOffset returns)

Options

options

optional setting for the reference instant

Option	Type	Default	Description
at?	string	The current instant.	The reference instant the zone is judged at, as an ISO 8601 instant string ending in Z, an offset or a bracketed zone. It only names an instant, as isValidInstant reads it: a bracketed zone is not validated and does not replace timeZone. An invalid value returns false.

Returns

boolean indicating whether the timeZone observes DST, or false on invalid input

Examples

import { hasDaylightSaving } from "@northguild/gmt/zoned/validate";

hasDaylightSaving("America/New_York", { at: "2024-01-15T12:00:00Z" }) // true

output

 

Source

zoned/validate/hasDaylightSaving.ts