Dates

In JavaScript, Date objects represent a single moment in time stored as a representation of the number of milliseconds since midnight on January 1st, 1970 UTC.

Note: This is different from a UNIX timestamp, which is the number of seconds since the Epoch (Midnight UTC on January 1st, 1970). Even though the representation of a Date is in UTC, the methods to fetch a date or its components work in the host system’s local time zone (which may differ from UTC).

Syntax

const myDate = new Date(dateValue);

The dateValue can be any of the following:

  • It can be a string representation of a date that is valid and IETF-compliant.
  • It can be another instance of the Date class.
  • It can be an integer that represents the date measured in milliseconds since January 1st, 1970 UTC.

The defined range for a Date value is between April 20, 271821 BCE to September 13, 275760 CE. Individual datetime elements can be defined in the Date() constructor with the following syntax:

const d = new Date(year, month, day, hour, minute, second, millisecond);

Date Format Types

In JavaScript, the following three date format types are commonly used:

Format Type Syntax
ISO Date (The International Standard) new Date("YYYY-MM-DDThh:mm:ss.sssZ)
Short Date new Date("YYYY-MM-DD") / new Date("YYYY/MM/DD")
Long Date new Date("Mon DD YYYY") ( or "DD Month YYYY")

With regard to ISO dates:

  • It is the only format that is strictly enforced while the others may vary in functionality depending on the browser.
  • “YYYY-MM-DD” or “YYYY/MM/DD” is the preferred format.
  • In the output, the T separates the the date from the time while the Z represents the UTC timezone.
  • Existing dates can be converted to ISO with the .toISOString() method.

With regard to short dates:

  • It is best practice to use leading zeros when referring to single-digit calendar days.
  • If using “YYYY-MM-DD” returns NaN the alternative format “YYYY/MM/DD” should be tested (and vice versa).

With regard to long dates:

  • The day and month can be in any order.
  • The month can either be abbreviated (“Mar”) or written in full (“March”).
  • Names are case-sensitive and commas can be ignored. (e.g. new Date("MONTH DD YYYY"))

Example

When used as a function, Date() returns the current date and time. When used as a constructor, Date() returns a new date object.

const now = Date();
console.log(now);
const then = new Date();
console.log(then);

The output from the snippet above would look similar to this:

Fri Apr 22 2022 17:59:19 GMT+0000 (Coordinated Universal Time)
2022-04-22T17:59:19.244Z

Codebyte Example

The example below defines a new Date object d. Many instance methods are used, which can be found at the bottom of this entry.

Dates

.getDate()
Called from an instance of the Date class, will return the date of the month. All return values will be integers between 1 and 31.
.getDay()
Called from an instance of the Date class, will return the day of the week.
.getHours()
Called from an instance of the Date class, will return the hour according to the local time.
.getMinutes()
Called from an instance of the Date class, will return the minutes according to the local time.
.getMonth()
Called from an instance of the Date class, will return the month of the year.
.getSeconds()
Called from an instance of the Date class, will return the seconds according to the local time.
.now()
Returns the current time in milliseconds. The milliseconds count begins at 1970/01/01 at 00:00:00 UTC.
.toISOString()
Returns a string representation of a date in an ISO 8601-compliant format.
.toLocaleDateString()
Returns a modified string from a given Date object, usually for events. It is translated to a specific language format according to an event locale and other options.
.toString()
Converts a Date object to a string.
.UTC()
Returns a number value representing the number of milliseconds between the specified date and January 1, 1970, 00:00:00, Universal Time Coordinated. Will always be called as Date.UTC() rather than called on an instance of date such as myDate.UTC().
.valueOf()
Returns the difference in milliseconds between the specified date and January 1, 1970 00:00:00 UTC.
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