The round() method of Temporal.ZonedDateTime instances returns a new Temporal.ZonedDateTime object representing this date-time rounded to the given unit.

Syntax

round(smallestUnit)
round(options)

Parameters

smallestUnit

A string representing the smallestUnit option. This is a convenience overload, so round(smallestUnit) is equivalent to round({ smallestUnit }), where smallestUnit is a string.

options

An object containing some or all of the following properties (in the order they are retrieved and validated):

roundingIncrement Optional

A number (truncated to an integer) representing the rounding increment in the given smallestUnit. Defaults to 1. For all values of smallestUnit except "day", the increment must be a divisor of the maximum value of the unit; for example, if the unit is hours, the increment must be a divisor of 24 and must not be 24 itself, which means it can be 1, 2, 3, 4, 6, 8, or 12. For "day", the increment must be 1.

roundingMode Optional

A string specifying how to round off the fractional part of smallestUnit. See Intl.NumberFormat(). Defaults to "halfExpand".

smallestUnit

A string representing the smallest unit to include in the output. The value must be one of the following: "day", "hour", "minute", "second", "millisecond", "microsecond", "nanosecond", or their plural forms. For units larger than "nanosecond", fractional parts of the smallestUnit will be rounded according to the roundingIncrement and roundingMode settings.

Return value

A new Temporal.ZonedDateTime object representing this date-time rounded to the given unit, where all units smaller than smallestUnit are zeroed out.

If smallestUnit is "day", the returned date-time will be the start of day of this date or the next day, depending on the roundingMode and the distance to these two instants. Otherwise, the rounding is first performed on its PlainDateTime (same as Temporal.PlainDateTime.prototype.round()), and then re-interpreted in the same time zone, with disambiguation: "compatible", offset: "prefer". See ambiguity and gaps from local time to UTC time and offset ambiguity.

Exceptions

RangeError

Thrown if any of the options is invalid.

Examples

Rounding off small units

const zdt = Temporal.ZonedDateTime.from(
  "2021-07-01T12:34:56.123456789[America/New_York]",
);
const nearestMillisecond = zdt.round("millisecond");
console.log(nearestMillisecond.toString()); // 2021-07-01T12:34:56.123-04:00[America/New_York]

const nearestHalfHour = zdt.round({
  smallestUnit: "minute",
  roundingIncrement: 30,
});
console.log(nearestHalfHour.toString()); // 2021-07-01T12:30:00-04:00[America/New_York]

const nextDay = zdt.round({ smallestUnit: "day", roundingMode: "ceil" });
console.log(nextDay.toString()); // 2021-07-02T00:00:00-04:00[America/New_York]

Ambiguity after rounding

It's possible that the rounded date-time is ambiguous in the given time zone. The ambiguity is always resolved using disambiguation: "compatible", offset: "prefer". Here's a quick example:

const zdt = Temporal.ZonedDateTime.from(
  "2024-03-10T01:00:00-05:00[America/New_York]",
);
const rounded = zdt.round({ smallestUnit: "hour", roundingIncrement: 2 });
// The result is supposed to be 2024-03-10T02:00:00-05:00[America/New_York],
// but this time does not exist. `disambiguation: "compatible"` tells us to move
// forward by 1 hour.
console.log(rounded.toString()); // 2024-03-10T03:00:00-04:00[America/New_York]

Specifications

Browser compatibility

See also

• Temporal.ZonedDateTime