Date() constructor

There are five basic forms for the Date() constructor:

No parameters

When no parameters are provided, the newly-created Date object represents the current date and time as of the time of instantiation. The returned date's timestamp is the same as the number returned by Date.now().

Time value or timestamp number

value

An integer value representing the timestamp (the number of milliseconds since midnight at the beginning of January 1, 1970, UTC — a.k.a. the epoch).

Date string

dateString

A string value representing a date, parsed and interpreted using the same algorithm implemented by Date.parse(). See date time string format for caveats on using different formats.

Date object

dateObject

An existing Date object. This effectively makes a copy of the existing Date object with the same date and time. This is equivalent to new Date(dateObject.valueOf()), except the valueOf() method is not called.

When one parameter is passed to the Date() constructor, Date instances are specially treated. All other values are converted to primitives. If the result is a string, it will be parsed as a date string. Otherwise, the resulting primitive is further coerced to a number and treated as a timestamp.

Individual date and time component values

Given at least a year and month, this form of Date() returns a Date object whose component values (year, month, day, hour, minute, second, and millisecond) all come from the following parameters. Any missing fields are given the lowest possible value (1 for day and 0 for every other component). The parameter values are all evaluated against the local time zone, rather than UTC. Date.UTC() accepts similar parameters but interprets the components as UTC and returns a timestamp.

If any parameter overflows its defined bounds, it "carries over". For example, if a monthIndex greater than 11 is passed in, those months will cause the year to increment; if a minutes greater than 59 is passed in, hours will increment accordingly, etc. Therefore, new Date(1990, 12, 1) will return January 1st, 1991; new Date(2020, 5, 19, 25, 65) will return 2:05 A.M. June 20th, 2020.

Similarly, if any parameter underflows, it "borrows" from the higher positions. For example, new Date(2020, 5, 0) will return May 31st, 2020.

year

Integer value representing the year. Values from 0 to 99 map to the years 1900 to 1999. All other values are the actual year. See the example.

monthIndex

Integer value representing the month, beginning with 0 for January to 11 for December.

day Optional

Integer value representing the day of the month. Defaults to 1.

hours Optional

Integer value between 0 and 23 representing the hour of the day. Defaults to 0.

minutes Optional

Integer value representing the minute segment of a time. Defaults to 0.

seconds Optional

Integer value representing the second segment of a time. Defaults to 0.

milliseconds Optional

Integer value representing the millisecond segment of a time. Defaults to 0.

Calling new Date() (the Date() constructor) returns a Date object. If called with an invalid date string, or if the date to be constructed will have a timestamp less than -8,640,000,000,000,000 or greater than 8,640,000,000,000,000 milliseconds, it returns an invalid date (a Date object whose toString() method returns "Invalid Date" and valueOf() method returns NaN).

Calling the Date() function (without the new keyword) returns a string representation of the current date and time, exactly as new Date().toString() does. Any arguments given in a Date() function call (without the new keyword) are ignored; regardless of whether it's called with an invalid date string — or even called with any arbitrary object or other primitive as an argument — it always returns a string representation of the current date and time.

To offer protection against timing attacks and fingerprinting, the precision of new Date() might get rounded depending on browser settings. In Firefox, the privacy.reduceTimerPrecision preference is enabled by default and defaults to 2ms. You can also enable privacy.resistFingerprinting, in which case the precision will be 100ms or the value of privacy.resistFingerprinting.reduceTimerPrecision.microseconds, whichever is larger.

For example, with reduced time precision, the result of new Date().getTime() will always be a multiple of 2, or a multiple of 100 (or privacy.resistFingerprinting.reduceTimerPrecision.microseconds) with privacy.resistFingerprinting enabled.

// reduced time precision (2ms) in Firefox 60
new Date().getTime();
// Might be:
// 1519211809934
// 1519211810362
// 1519211811670
// …

// reduced time precision with `privacy.resistFingerprinting` enabled
new Date().getTime();
// Might be:
// 1519129853500
// 1519129858900
// 1519129864400
// …

The following examples show several ways to create JavaScript dates:

const today = new Date();
const birthday = new Date("December 17, 1995 03:24:00"); // DISCOURAGED: may not work in all runtimes
const birthday = new Date("1995-12-17T03:24:00"); // This is standardized and will work reliably
const birthday = new Date(1995, 11, 17); // the month is 0-indexed
const birthday = new Date(1995, 11, 17, 3, 24, 0);
const birthday = new Date(628021800000); // passing epoch timestamp

If the Date() constructor is called with one parameter which is not a Date instance, it will be coerced to a primitive and then checked whether it's a string. For example, new Date(undefined) is different from new Date():

console.log(new Date(undefined)); // Invalid Date

This is because undefined is already a primitive but not a string, so it will be coerced to a number, which is NaN and therefore not a valid timestamp. On the other hand, null will be coerced to 0.

console.log(new Date(null)); // 1970-01-01T00:00:00.000Z

Arrays would be coerced to a string via Array.prototype.toString(), which joins the elements with commas. However, the resulting string for any array with more than one element is not a valid ISO 8601 date string, so its parsing behavior would be implementation-defined. Do not pass arrays to the Date() constructor.

console.log(new Date(["2020-06-19", "17:13"]));
// 2020-06-19T17:13:00.000Z in Chrome, since it recognizes "2020-06-19,17:13"
// "Invalid Date" in Firefox

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