JavaScript
Dates
The Date
object lets you work with dates (years, months, days, hours, minutes, seconds,
and milliseconds)
JavaScript
Date Formats
A JavaScript
date can be written as a string:
Tue Apr23
2019 15:56:07 GMT+0760 (IST)
or as a
number:
1524565687888
Dates
written as numbers, specifies the number of milliseconds since January 1, 1970,
00:00:00.
Displaying
Dates
In this
tutorial we use a script to display dates inside a <p> element with
id="demo":
Example
<p
id="demo"></p>
<script>
document.getElementById("demo").innerHTML
= Date();
</script>
»
The script
above says: assign the value of Date() to the content (innerHTML) of the
element with id="demo".
You will
learn how to display a date, in a more readable format, at the bottom of this
page.
Creating
Date Objects
The Date
object lets us work with dates.
A date
consists of a year, a month, a day, an hour, a minute, a second, and
milliseconds.
Date objects
are created with the new Date() constructor.
There are 4
ways of initiating a date:
new Date()
new
Date(milliseconds)
new
Date(dateString)
new
Date(year, month, day, hours, minutes, seconds, milliseconds)
Using new
Date(), creates a new date object with the current date and time:
Example
<script>
var
d = new Date();
document.getElementById("demo").innerHTML
= d;
</script>
»
Using new
Date(date string), creates a new date object from the specified date and time:
Example
<script>
var
d = new Date("October 13, 2014 11:13:00");
document.getElementById("demo").innerHTML
= d;
</script>
»
Valid date
strings (date formats) are described in the next chapter.
Using new
Date(number), creates a new date object as zero time plus the number.
Zero time is
01 January 1970 00:00:00 UTC. The number is specified in milliseconds:
Example
<script>
var
d = new Date(86400000);
document.getElementById("demo").innerHTML
= d;
</script>
»
JavaScript
dates are calculated in milliseconds from 01 January, 1970 00:00:00 Universal
Time (UTC). One day contains 86,400,000 millisecond.
Using new
Date(7 numbers), creates a new date object with the specified date and time:
The 7
numbers specify the year, month, day, hour, minute, second, and millisecond, in
that order:
Example
<script>
var
d = new Date(99, 5, 24, 11, 33, 30, 0);
document.getElementById("demo").innerHTML
= d;
</script>
»
Variants of
the example above let us omit any of the last 4 parameters:
Example
<script>
var
d = new Date(99, 5, 24);
document.getElementById("demo").innerHTML
= d;
</script>
»
JavaScript
counts months from 0 to 11. January is 0. December is 11.
Date
Methods
When a Date
object is created, a number of methods allow you to operate on it.
Date methods
allow you to get and set the year, month, day, hour, minute, second, and
millisecond of objects, using either local time or UTC (universal, or GMT)
time.
Date methods
are covered in a later chapter.
Displaying
Dates
When you
display a date object in HTML, it is automatically converted to a string, with
the toString() method.
Example
<p
id="demo"></p>
<script>
d
= new Date();
document.getElementById("demo").innerHTML
= d;
</script>
Is
the same as:
<p
id="demo"></p>
<script>
d
= new Date();
document.getElementById("demo").innerHTML
= d.toString();
</script>
»
The
toUTCString() method converts a date to a UTC string (a date display standard).
Example
<script>
var
d = new Date();
document.getElementById("demo").innerHTML
= d.toUTCString();
</script>
»
The
toDateString() method converts a date to a more readable format:
Example
<script>
var
d = new Date();
document.getElementById("demo").innerHTML
= d.toDateString();
</script>
»
Date objects
are static. The computer time is ticking, but date objects, once created, are
not.
Time Zones
When setting
a date, without specifying the time zone, JavaScript will use the browser's
time zone.
When getting
a date, without specifying the time zone, the result is converted to the
browser's time zone.
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