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).

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:

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"))

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

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

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