Temporal Date Formats
Temporal dates can be serialized as strings in different ways:
| Name | Description |
|---|---|
| ISO 8601 | International standard |
| RFC 3339 | Internet standard |
| RFC 9557 | Temporal standard |
| toString() | JavaScript method |
| toLocaleString() | JavaScript method |
| Intl.DateTimeFormat() | JavaScript method |
The ISO 8601 Standard
ISO 8601 is the International Standard for representing dates and times.
ISO 8601 is a machine-readable format (YYYY-MM-DD) designed to eliminate confusion caused by regional date variations.
It follows a "largest to smallest" hierarchy (year, month, day, hour, minute, second) which ensures that dates are alphabetically sortable.
ISO 8601 avoids confusion between regional formats like:
- MM/DD/YYYY (US format)
- DD/MM/YYYY (European format)
ISO 8601 Strings
An ISO 8601 string represents dates and times in a structured and sortable format:
| Format | Description |
|---|---|
| YYYY | A 4-digit year |
| MM | 2-digit month |
| DD | 2-digit day |
| T | Separator for time |
| hh | Hour |
| mm | Minute |
| ss | Second |
| Z | Zero (Zulu) time Zone |
Examples
| Format | Format | Example |
|---|---|---|
| Date (4-digit year) | YYYY-MM-DD | 2026-05-17 |
| Time (24 hour) | hh:mm:ss | 14:30:00 |
| Date & Time | YYYY-MM-DDThh:mm:ssZ | 2026-05-17T14:30:00Z |
| Date & Time UTC | YYYY-MM-DDThh:mm:ssZ | 2026-05-17T14:30:00Z |
| Date & Time + Offset | YYYY-MM-DDThh:mm:ss±hh:mm | 2026-05-17T14:30:00+01:00 |
| Week Date | YYYY-Www-D | 2026-W20-7 |
| Ordinal (001-366) | YYYY-nnn | 2026-074 |
| Duration format | PnYnMnDTnHnMnS | P3Y6M4DT12H30M5S |
The RF 3339 Format
RFC 3339 is the Internet Standard for Date and Time Formats.
It is a Subset of ISO 8601 designed for use in Internet protocols.
| Feature | Description |
|---|---|
| Format | Follows the ISO pattern YYYY-MM-DDThh:mm:ssZ |
| Time Zones | Requires a Z for time zone UTC or an offset (+05:00) |
| T Separator | Allows the "T" separator to be replaced by " " for readability |
| Web APIs | The default format for timestamps in JSON and RESTful APIs |
| Programming | Modern languages have bult-in support like toISOString() |
The RFC 9557 Format
RFC 9557 is the primary format used by JavaScript Temporal to represent dates.
| Feature | ISO 8601 | RFC 3339 | RFC 9557 |
|---|---|---|---|
| Format | YYYY-MM-DDThh:mm:ssZ | Same | Same |
| UTC Offset | Yes | Yes | Yes |
| IANA Zone | No | No | [America/New_York] |
| Calendar | ISO only | ISO only | [u-ca=hebrew] |
| Compabile | The Standard | Subset of ISO 8601 | Extends 3339 |
RFC 9557 was created because ISO 8601 and RFC 3339 did not contain enough information to determine the geographical time zone or handle future DST (Daylight Saving Time) changes.
Format with toString()
The toString() method returns a standard ISO string.
Example
const date = Temporal.PlainDate.from("2026-02-17");
let text = date.toString();
Result:
2026-02-17
Format PlainTime
A PlainTime value is formatted as a time string.
Example
const time = Temporal.PlainTime.from("14:30:15");
let text = time.toString();
Result:
14:30:15
Format PlainDateTime
A PlainDateTime value includes both date and time.
Example
const dateTime = Temporal.PlainDateTime.from("2026-02-17T14:30:15");
let text = dateTime.toString();
Result:
2026-02-17T14:30:15
Format ZonedDateTime
A ZonedDateTime string includes the offset and time zone.
Example
const zoned = Temporal.ZonedDateTime.from(
"2026-02-17T14:30:15+01:00[Europe/Oslo]");
let text = zoned.toString();
Result:
2026-02-17T14:30:15+01:00[Europe/Oslo]
Remove Smaller Time Units
You can control how much detail to include.
For example, you can hide seconds or milliseconds.
Example
const time = Temporal.PlainTime.from("14:30:15.123");
let text = time.toString({ smallestUnit: "minute" });
Result:
14:30
Control Fractional Digits
You can choose how many decimal digits to include.
Example
const time = Temporal.PlainTime.from("14:30:15.123456789");
let text = time.toString({ fractionalSecondDigits: 3 });
Result:
14:30:15.123
Format with toLocaleString()
Use toLocaleString() to format a value using the user's locale.
Example
const date = Temporal.PlainDate.from("2026-02-17");
console.log(date.toLocaleString("en-US"));
console.log(date.toLocaleString("de-DE"));
The output depends on the chosen locale.
Format with Intl.DateTimeFormat
Use Intl.DateTimeFormat when you need more control over the output.
Example
const date = Temporal.PlainDate.from("2026-02-17");
const formatter = new Intl.DateTimeFormat("en-US", {
year: "numeric",
month: "long",
day: "numeric"
});
let text = formatter.format(date);
Result:
February 17, 2026
Format Date and Time Together
You can format a PlainDateTime with both date and time options.
Example
const dateTime = Temporal.PlainDateTime.from("2026-02-17T14:30:15");
const formatter = new Intl.DateTimeFormat("en-GB", {
year: "numeric",
month: "short",
day: "numeric",
hour: "2-digit",
minute: "2-digit"
});
let text = formatter.format(dateTime);
Format ZonedDateTime in a Locale
When formatting a ZonedDateTime, the time zone is included in the calculation.
Example
const zoned = Temporal.ZonedDateTime.from(
"2026-02-17T14:30:15+01:00[Europe/Oslo]");
const formatter = new Intl.DateTimeFormat("en-US", {
dateStyle: "full",
timeStyle: "short"
});
let text = formatter.format(zoned);
When to Use Each Method
Use
toString()for standard ISO output.Use
toLocaleString()for simple local formatting.Use
Intl.DateTimeFormatfor custom formatting.
