d
.
getHours
()
// 17
d
.
getMinutes
()
// 0
d
.
getSeconds
()
// 0
d
.
getMilliseconds
()
// 0
// there are allso UTC equivalents to the above:
d
.
getUTCFullYear
()
// 1815
d
.
getUTCMonth
()
// 9 - October
d
.
getUTCDate
()
// 10
// ...etc.
If you’re using Moment.js, you’ll find little need to work with individual components,
but it’s good to know that they’re there.
Comparing Dates
For simple date comparisons—does date A come after date B or vice versa?—you can
use JavaScript’s built-in comparison operators. Remember that
Date
instances store
the date as a number, so the comparison operators simply work on the numbers:
const
d1
=
new
Date
(
1996
,
2
,
1
);
const
d2
=
new
Date
(
2009
,
4
,
27
);
d1
>
d2
// false
d1
<
d2
// true
Date Arithmetic
Because dates are just numbers, you can subtract dates to get the number of millisec‐
onds between them:
const
msDiff
=
d2
-
d1
;
// 417740400000 ms
const
daysDiff
=
msDiff
/
1000
/
60
/
60
/
24
;
// 4834.96 days
This property also makes it easy to sort dates using
Array.prototype.sort
:
const
dates
=
[];
// create some random dates
const
min
=
new
Date
(
2017
,
0
,
1
).
valueOf
();
const
delta
=
new
Date
(
2020
,
0
,
1
).
valueOf
()
-
min
;
for
(
let
i
=
0
;
i
<
10
;
i
++
)
dates
.
push
(
new
Date
(
min
+
delta
*
Math
.
random
()));
// dates are random and (probably) jumbled
// we can sort them (descending):
dates
.
sort
((
a
,
b
)
=>
b
-
a
);
// or ascending:
dates
.
sort
((
a
,
b
)
=>
a
-
b
);
Moment.js brings many powerful methods for doing common date arithmetic, allow‐
ing you to add or subtract arbitrary units of time:
226 | Chapter 15: Date and Time
