Sep 22 2009

Exploring Javascript Date Math

Dates are a peculiar type of data that we have to work with. In some sense, they aren’t a number, but instead are a combination of month, day, and year. But at the same time, in most programming languages, they are fundamentally a number: the number of seconds since the Unix epoch.

In our heads, it’s quite easy to do Date math. Moving forward a week requires just adding 7 days. How would you do that with a Javascript Date? Programming languages usually try to make working with Dates as logical as it is to do in our heads. So I a little surprised at an irregularity in using Dates in my math.

Though to be fair, it seems I could blame it on Javascript’s decision to make the addition operator and concatenation operator the same symbol. I only did some quick, sloppy tests in Firefox 3.5 and Chrome 3.

Adding to Dates

I first wanted to move a date forward a week, so I thought that perhaps I could add 7 days worth of milliseconds to the date to get next week. Go ahead and try it:

new Date() + (1000 * 60 * 60 * 24 * 7)

The value is a string: “Fri Sep 18 2009 16:37:01 GMT-0700 (Pacific Daylight Time)604800000”. Yup, there it is! Our week’s worth of milliseconds, attached on the end of a Date string. It would seem that Javascript decides that the Date should be evaluated as a string instead of a number at that point.

How To Add

In order to add to Dates, I had to create a new Date, using the value returned from getTime and adding milliseconds to it, and using that new (humongous) number in the constructor.

var nextWeek = new Date(today.getTime() + 1000 * 60 * 60 * 24 * 7);

This is because the Date’s toString method is being evaluated instead of it’s valueOf. We can also overcome that with an additional plus sign.

+(new Date()) + (1000 \* 60 \* 60 \* 24 \* 7)

Also to keep in mind here, is that unlike in, say, PHP where you provide seconds to the date function, in Javascript, you must provide milliseconds, hence the extra 1000 in there.

Subtracting from Dates

Now, until I remembered that + means to different operations in Javascript, I was rather shocked that I could easily subtract from Dates, but not add.

new Date() - (1000 * 60 * 60 * 24 * 7)

Result is a number: 1252714884521. This does the proper math, but not everything I had hoped and dreamed for. No matter, receiving a number is certainly a result I could expect, as opposed to the string joining from addition.

  • #javascript