JavaScript Dates

In the JavaScript Dates topic, you can learn how JavaScript supports date and you can create it. Date objects represent a single moment in time in a platform-independent format.

Time-zones

There are hundreds of time zones in our world. In JavaScript, we only care about two—Local Time and Coordinated Universal Time (UTC).

  • Local time refers to the timezone your computer is in.
  • UTC is synonymous with Greenwich Mean Time (GMT) in practice.

By default, almost every date method in JavaScript (except one) gives you a date/time in local time. You only get UTC if you specify UTC.

Creating JavaScript dates

We can create a date with new Date(). There are possible ways to use new Date():

The date-String

In the date-string method, you create a date by passing a date-string into new Date.

<!DOCTYPE html>
<html>
<body>

<script>
var d = new Date();
document.write(d);
</script>

</body>
</html>

Output:

JavaScript Dates

Creating JavaScript Dates with & without Arguments

There are seven arguments pass in the new Date() method. As you go left to right, you insert values in decreasing magnitude: year, month, day, hours, minutes, seconds, and milliseconds.

new Date()

Without arguments – create a Date object for the current date and time:

<!DOCTYPE html>
<html>
<body>

<script>
let n = new Date();
alert(n); 
</script>

</body>
</html>

Output:

image 17

new Date(milliseconds)

Create a Date object with the time equal to a number of milliseconds (1/1000 of a second) passed after the Jan 1st of 1970 UTC+0. 

<!DOCTYPE html>
<html>
<body>

<script>
let Jan01_1970 = new Date(0);
alert( Jan01_1970 );

let Jan02_1970 = new Date(24 * 3600 * 1000);
alert( Jan02_1970 ); 
</script>

</body>
</html>

Output:

image 18

An integer number representing the number of milliseconds that have passed since the beginning of 1970 is called a timestamp.

It’s a lightweight numeric representation of a date. We can always create a date from a timestamp using new Date(timestamp) and convert the existing Date object to a timestamp using the date.getTime() method (see below).

Dates before 01.01.1970 have negative timestamps, e.g.:

<!DOCTYPE html>
<html>
<body>

<script>
// 31 Dec 1969
let Dec31_1969 = new Date(-24 * 3600 * 1000);
alert( Dec31_1969 );
</script>

</body>
</html>

Output:

image 19

new Date(datestring)

If there is a single argument, and it’s a string, then it is parsed automatically. The algorithm is the same as Date.parse uses, we’ll cover it later. 

<!DOCTYPE html>
<html>
<body>

<script>
let date = new Date("2017-01-26");
alert(date);
</script>

</body>
</html>

Output:

image 20

new Date(year, month, date, hours, minutes, seconds, ms)

Create the date with the given components in the local time zone. Only the first two arguments are obligatory.

  • The year must have 4 digits: 2013 is okay, 98 is not.
  • The month count starts with 0 (Jan), up to 11 (Dec).
  • The date parameter is actually the day of month, if absent then 1 is assumed.
  • If hours/minutes/seconds/ms is absent, they are assumed to be equal 0.

For instance:

The maximal precision is 1 ms (1/1000 sec):