Temporal.ZonedDateTime.prototype.startOfDay()

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The startOfDay() method of Temporal.ZonedDateTime instances returns a Temporal.ZonedDateTime object representing the first instant of this date in the time zone. It usually has a time of 00:00:00, but may be different if the midnight doesn't exist due to offset changes, in which case the first time that exists is returned.

It is equivalent to calling withPlainTime() with no arguments.

Syntax

js
startOfDay()

Parameters

None.

Return value

A Temporal.ZonedDateTime object with instant t, such that:

  • The date at t is different from the date one nanosecond before t.
  • The date at t is the same as the date of this.

Examples

Using startOfDay()

js
// In the US, DST transitions happen at 2am, so the midnight exists
const dt = Temporal.ZonedDateTime.from(
  "2024-03-10T12:00:00-04:00[America/New_York]",
);
console.log(dt.startOfDay().toString()); // "2024-03-10T00:00:00-05:00[America/New_York]"

// In Brazil, DST transitions happened at midnight, so the midnight didn't exist
const dt2 = Temporal.ZonedDateTime.from(
  "2015-10-18T12:00-02:00[America/Sao_Paulo]",
);
console.log(dt2.startOfDay().toString()); // "2015-10-18T01:00:00-02:00[America/Sao_Paulo]"

Specifications

Specification
Temporal proposal
# sec-temporal.zoneddatetime.prototype.startofday

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
startOfDay
Experimental

Legend

Tip: you can click/tap on a cell for more information.

No support
No support
Experimental. Expect behavior to change in the future.
See implementation notes.
User must explicitly enable this feature.

See also