747

Assigning a Date variable to another one will copy the reference to the same instance. This means that changing one will change the other.

How can I actually clone or copy a Date instance?

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8 Answers 8

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1070

Use the Date object's getTime() method, which returns the number of milliseconds since 1 January 1970 00:00:00 UTC (epoch time):

var date = new Date(); var copiedDate = new Date(date.getTime());

In Safari 4, you can also write:

var date = new Date(); var copiedDate = new Date(date);

...but I'm not sure whether this works in other browsers. (It seems to work in IE8).

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12 Comments

JSON for this snippet? Sounds like these people should get their basics clear... Like mistaking jQuery for JavaScript DOM.
Another way to write this nice solution would be to extend the Date prototype: Date.prototype.clone = function() { return new Date(this.getTime()); }; Which you could then use as copiedDate = date.clone();
new Date(date) same as new Date(date.getTime()), because JS will try to call date.valueOf() when it need a number, and date.valueOf() is same as date.getTime(), reference Date.valueOf Object.valueOf
Do not use new Date(date), use new Date(date.getTime() or new Date(date.valueOf) instead since the first way can lead to differences between the dates in at least Firefox and IE (not Chrome). For example using toISOString() on the both dates in Firefox generates "2015-04-21T04:56:42.000Z" and "2015-04-21T04:56:42.337Z".
new Date(dateObject) is correct. See description of Date(value) constructor where explicit rules for date object is defined. In the older specs the behavior was to call ToNumber on the argument if it wasn't a string so either way this should work in any browser that implements the behavior correctly.
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189

This is the cleanest approach

let date = new Date() let copyOfDate = new Date(date.valueOf()) console.log(date); console.log(copyOfDate);

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5 Comments

The "valueOf()" method for "Date" objects produces the same result as its "getTime()" method (the number of milliseconds since epoch time).
@Steve: true, but getTime() could "looks" like it only returns the time and not include the date as well hence my reference to "cleanest". Frankly the Date type in Javascript is a bit of disaster zone, it should never have been mutable in the first place.
@AnthonyWJones: Right, I see what you mean.
I agree that .valueOf() is more clear. Sometimes I forget and use .getMilliseconds() b/c to me that sounds like it means mean milliseconds since epoch time.
+1 to Steve Harrison: I was wondering if that was the case, thanks for the clarification.
96

Update for 2021:

In one respect the notion of cloning a Date object sounds grander than it really is. As far as I can tell, there’s only one piece of instance data, and that is the stored time. What we’re really doing is making a new object with the same time.

Whatever may have been the case in the past, the new Date() constructor definitely accepts a Date object as a single argument:

const date = new Date(); // With or without an argument. const date2 = new Date(date); // Clone original date.

The Specification at https://tc39.es/ecma262/multipage/numbers-and-dates.html#sec-date at step 4(b) indicates that a Date object is definitely acceptable, and that this is equivalent to new Date(date.valueOf()), as suggested by some of the answers above. As I said, all you’re really doing is making a new Date object with the same time as the other.

You’ll also find that the documentation at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/Date has been updated to include this.

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41 

var orig = new Date(); var copy = new Date(+orig); console.log(orig, copy);

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7 Comments

I like this solution the best.
Very precise and clean :)
Except you'll have to explain what that magic + is doing to anyone but JS experts.
:) + sign is unaray operator here. It means new Date( Number(orig)) . More here: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…
Only works if orig is not a string such as 2020-09-02T06:49:51.4000654Z
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15

Simplified version:

Date.prototype.clone = function () { return new Date(this.getTime()); }
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4 Comments

thou shalt not mess with built-in objects
Thou shalt not mess with objects thou dost not owneth. You should make a new copy and call it SuperDate or something, withing your scope. Lots of hard to test for bugs are caused by object functionality changing unexpectedly.
This would work, but for maintainability reasons, This approach would be considered as a code smell. I have written an approach that I usually use in my coding: actuts.wordpress.com/2017/01/10/…
Also I don't see this need for trying to add methods to built-ins in the first place. Study functional programming and discover why a good old-fashioned function is actually much more powerful than methods on the object itself. It's also shorter: const cloneDate = d => new Date(d.getTime()).
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I found out that this simple assignmnent also works:

dateOriginal = new Date(); cloneDate = new Date(dateOriginal);

But I don't know how "safe" it is. Successfully tested in IE7 and Chrome 19.

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3 Comments

Do not use new Date(date), use new Date(date.getTime() or new Date(date.valueOf) instead since the first way can lead to differences between the dates in at least Firefox and IE (not Chrome). For example using toISOString() on the both dates in Firefox generates "2015-04-21T04:56:42.000Z" and "2015-04-21T04:56:42.337Z".
That may have been true on old versions of firefox, but on the latest version, new Date(date) works fine. Try it: let date = '2015-04-21T04:56:42.337Z'; new Date(date).toISOString(). Result is the same as input: "2015-04-21T04:56:42.337Z"
To anyone reading this nowadays, this works, please ignore old comments stating that .getTime must be used.
0 
function cloneMyDate(oldDate){ var newDate = new Date(this.oldDate); }

I was passing oldDate to function and generating newDate from this.oldDate, but it was changing this.oldDate also.So i used the above solution and it worked.

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-4

For chrome it works:

//Thu Nov 03 2022 11:43:00 const date = new Date(2022,10,03,11,43,0); const date2 = new Date(date);

now changing date2 will have no impact on the date

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2 Comments

Please learn how to use markdown and adjust your answer accordingly.
The formatting isn’t the only problem with this answer. It contains something that has already been covered by all previous answers.

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