adding / subtracting time in JS

Question

Lately I was playing with JS and I found something interesting. That is what I wrote into chrome console:

today = new Date()
-> Mon Apr 29 2013 13:06:01 GMT+0200 (CEST)
DAY = 1000 * 3600 * 24
-> 86400000
today - 2 * DAY
-> 1367060761452
today + 2 * DAY
-> "Mon Apr 29 2013 13:06:01 GMT+0200 (CEST)172800000"

And I am wondering why am I getting different types of answer depending on the type of operation - adding / subtracting. When I do something like that:

today - (-2) * DAY

everything is fine. Is there any ideology, or is it a bug?

That’s because + can be the addition or the string concatenation operator, depending on context. (If you have a string value and try to “add” anything to it, the second argument gets usually converted to string and concatenation happens instead of addition.) — C3roe
– C3roe, Commented Apr 30, 2013 at 9:04

Bartosz Marcinkowski · Accepted Answer · 2013-04-30 09:04:33Z

3

today + 2 * DAY uses concatenation of strings. If you want do it properly, use today.getTime().

Example:

tomorrow = new Date()
tomorrow.setTime(today.getTime() + DAY)

answered Apr 30, 2013 at 9:04

Bartosz Marcinkowski

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Dagg Nabbit · Accepted Answer · 2013-04-30 09:21:19Z

This is a bit tricky to find in the spec, because it's not with the rest of the Date stuff.

If you take a look at section 11.6.1, "The Addition operator," you'll find the following note:

NOTE 1 No hint is provided in the calls to ToPrimitive in steps 5 and 6. All native ECMAScript objects except Date objects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if the hint String were given. Host objects may handle the absence of a hint in some other manner.

In context, that means using the addition operator (+) with a Date object will use the string value rather than the numeric value. In this sense, Date objects are special and unlike any other kind of objects.

Note that there is no such exception for the subtraction operator, as it is unambiguous — it only works for numeric subtraction; it doesn't operate on strings.

Also note that this applies to the addition operator, a + with operands on both sides. The unary "plus" operator does not work like this, so +myDateObj with nothing on the left hand side will result in the numeric value.

xdazz · Accepted Answer · 2013-04-30 09:03:53Z

2

For -, that is the minus operator, using the .valueOf method of the date object.

While for +, that is first considered as string concatenation.

today - 2 * DAY is considered as today.valueOf() - 2 * DAY

today + 2 * DAY is considered as today.toString() + 2 * DAY

So if you want to use math operation on date object, use today.getTime() instead of just today.

answered Apr 30, 2013 at 9:03

xdazz

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Comments

Kamal Pundir · Accepted Answer · 2013-04-30 11:42:46Z

0

I think add and subtract days to javascript dates like this

var today= new Date();
var addDay=30;
today.setDate(today.getDate()+addDay);

Now today is pointing the next 30th date from today

edited Apr 30, 2013 at 11:42

answered Apr 30, 2013 at 9:41

Kamal Pundir

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