JavaScript Date Object Comparison

Question

When comparing date objects in Javascript I found that even comparing the same date does not return true.

 var startDate1 = new Date("02/10/2012");
 var startDate2 = new Date("01/10/2012");
 var startDate3 = new Date("01/10/2012");
 alert(startDate1>startDate2); // true
 alert(startDate2==startDate3); //false

How could I compare the equality of these dates? I am interested in utilizing the native Date object of JS and not any third party libraries since its not appropriate to use a third party JS just to compare the dates.

Excellent example of how this is poorly designed in JavaScript. — devios1, Dec 02 '12 at 20:11

score 143 · Accepted Answer · edited Sep 23 '20 at 13:42

143

That is because in the second case, the actual date objects are compared, and two objects are never equal to each other. Coerce them to number:

 alert( +startDate2 == +startDate3 ); // true

If you want a more explicity conversion to number, use either:

 alert( startDate2.getTime() == startDate3.getTime() ); // true

or

 alert( Number(startDate2) == Number(startDate3) ); // true

Oh, a reference to the spec: §11.9.3 The Abstract Equality Comparison Algorithm which basically says when comparing objects, obj1 == obj2 is true only if they refer to the same object, otherwise the result is false.

edited Sep 23 '20 at 13:42

rioki

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answered Sep 30 '11 at 06:47

RobG

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I'm upped answer, but is more right way to use _strict equals_ operator `===` in your examples? – Andrew D. Sep 30 '11 at 06:53
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@AndrewD. using strict equals in this particular case doesn't make any difference on the results, this is because the equals operator in the examples, is always dealing with operands of the same type, @RobG is converting the values explicitly to Number (example 1 and 3) or in the example 2, we know that `Date.prototype.getTime` will always return a Number... – Christian C. Salvadó Sep 30 '11 at 07:25
@RobG: When you say " `obj1 == obj2` is true only if they refer to the same object", does that mean that the comparison (if both operands are objects) does not test for equality, but rather identity? Is my understanding of this correct? – Russell Dias Dec 06 '11 at 02:02
@RussellDias—yes. Where an object is assigned to a variable, the variable holds a reference to the object, so more than one variable can reference the same object. The rules for the abstract equality algorithm are quite long, but for objects it's pretty simple. – RobG Dec 06 '11 at 04:58
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FYI, there is a significant performance difference between these approaches: http://jsperf.com/date-equality-comparison – Nick Zalutskiy Jan 13 '13 at 01:13
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@Nick—even the slowest version takes less than a microsecond to run, so while there are comparative differences, in absolute terms the performance difference is negligible. The OP should just chose whichever approach suits best, likely using `getTime` is best for clarity (and happens to be fastest in the browsers I tested too). – RobG Jan 14 '13 at 05:30
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@RobG You know, you are absolutely right. =) I was writing a library and did a test "just cuz." In real software it makes no difference whatsoever. – Nick Zalutskiy Jan 15 '13 at 05:12
@NickZalutskiy Thanks. I was going to right a similiar jspref but thought I'd hit up the comments first. ms' matter. – matchew Feb 20 '14 at 19:43
Could someone point me in the right direction to learn more about adding `+` in front of variables for comparison? – Hanna Jul 23 '14 at 20:45
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@Johannes—see [*What is unary + used for in Javascript?*](http://stackoverflow.com/questions/9081880/what-is-unary-used-for-in-javascript) and [*The unary + operator*](http://xkr.us/articles/javascript/unary-add/). – RobG Jul 23 '14 at 23:09

score 27 · Answer 2 · answered Sep 30 '11 at 06:48

Compare dates using getTime() returning number of milliseconds from epoch (i.e. a number):

var startDate1 = new Date("02/10/2012");
var startDate2 = new Date("01/10/2012");
var startDate3 = new Date("01/10/2012");
alert(startDate1.getTime() > startDate2.getTime()); // true
alert(startDate2.getTime() == startDate3.getTime()); //true

Also consider using Date constructor taking explicit year/month/date number rather then relying on string representation (see: Date.parse()). And remember that dates in JavaScript are always represented using client (browser) timezone.

+1 for the comment on using strings as the argument to Date(). — RobG, Sep 30 '11 at 07:56

score 18 · Answer 3 · answered Sep 30 '11 at 08:38

18

You do not need to use the getTime method- you can subtract a date object from another date object. It will return the milliseconds difference(negative, if the second is a later date)

var startDate1 = new Date("02/10/2012");
var startDate2 = new Date("01/10/2012");

var diff= (startDate1 -startDate2)

// evaluates to 0 if the dates have the same timestamp

answered Sep 30 '11 at 08:38

kennebec

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+1 simple and elegant, imo preferred solution: this uses built-in Date evaluation functionality without the need of coercion to a numeric timestamp – gdibble Jan 05 '16 at 22:25

gion_13 · Answer 4 · 2016-01-26T07:43:22.137

5

you can compare the actual milliseconds :

alert(startDate2.getTime() === startDate3.getTime());

edited Jan 26 '16 at 07:43

answered Sep 30 '11 at 06:50

gion_13

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score 0 · Answer 5 · answered Jun 27 '19 at 13:56

You can also use the function valueOf()

 var startDate1 = new Date("02/10/2012").valueOf();
 var startDate2 = new Date("01/10/2012").valueOf();
 var startDate3 = new Date("01/10/2012").valueOf();
 alert(startDate1>startDate2); // 1326150000000 > 1328828400000   true
 alert(startDate2==startDate3); // 1328828400000 > 1326150000000  false

JavaScript Date Object Comparison

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