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.

Time value or timestamp number

value

An integer value representing the number of milliseconds since January 1, 1970, 00:00:00 UTC (the ECMAScript epoch, equivalent to the UNIX epoch), with leap seconds ignored. Keep in mind that most UNIX Timestamp functions are only accurate to the nearest second.

Date string

dateString

A string value representing a date, in a format recognized by the Date.parse() method. (The ECMA262 spec specifies a simplified version of ISO 8601, but other formats can be implementation-defined, which commonly include IETF-compliant RFC 2822 timestamps.)

Note: When parsing date strings with the Date constructor (and Date.parse, they are equivalent), always make sure that the input conforms to the ISO 8601 format (YYYY-MM-DDTHH:mm:ss.sssZ) — the parsing behavior with other formats is implementation-defined and may not work across all browsers. Support for RFC 2822 format strings is by convention only. A library can help if many different formats are to be accommodated.

Date-only strings (e.g. "1970-01-01") are treated as UTC, while date-time strings (e.g. "1970-01-01T12:00") are treated as local. You are therefore also advised to make sure the input format is consistent between the two types.

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 will be converted to a primitive (calling [@@toPrimitive]("default"), valueOf(), and toString(), in that order). 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.

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. The default is 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. The default is 0 minutes past the hour.

seconds Optional

Integer value representing the second segment of a time. The default is 0 seconds past the minute.

milliseconds Optional

Integer value representing the millisecond segment of a time. The default is 0 milliseconds past the second.

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 UNIX timestamp less than -8,640,000,000,000,000 or greater than 8,640,000,000,000,000 milliseconds, it returns a Date object whose toString() method returns the literal string Invalid Date.

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 wth any arbitrary object or other primitive as an argument — it always returns a string representation of the current date and time.

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 ISO-8601-compliant 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. Date()

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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