Temporal.ZonedDateTime.prototype.startOfDay()

The startOfDay() method of {{jsxref("Temporal.ZonedDateTime")}} instances returns a {{jsxref("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 {{jsxref("Temporal/ZonedDateTime/withPlainTime", "withPlainTime()")}} with no arguments.

Syntax#

startOfDay()

Parameters#

None.

Return value#

A {{jsxref("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()#

// 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#

{{Specifications}}

Browser compatibility#

{{Compat}}

See also#

• {{jsxref("Temporal.ZonedDateTime")}}
• {{jsxref("Temporal/ZonedDateTime/with", "Temporal.ZonedDateTime.prototype.with()")}}
• {{jsxref("Temporal/ZonedDateTime/add", "Temporal.ZonedDateTime.prototype.add()")}}
• {{jsxref("Temporal/ZonedDateTime/subtract", "Temporal.ZonedDateTime.prototype.subtract()")}}
• {{jsxref("Temporal/ZonedDateTime/hour", "Temporal.ZonedDateTime.prototype.hour")}}
• {{jsxref("Temporal/ZonedDateTime/hoursInDay", "Temporal.ZonedDateTime.prototype.hoursInDay")}}
• {{jsxref("Temporal/ZonedDateTime/timeZoneId", "Temporal.ZonedDateTime.prototype.timeZoneId")}}
• {{jsxref("Temporal/ZonedDateTime/getTimeZoneTransition", "Temporal.ZonedDateTime.prototype.getTimeZoneTransition()")}}