GWT and JavaScript Date hell

By Olivier Gérardin

You’d think that in 2010, date processing is something that is easily done in Java (and hence GWT), in a consistent cross-platform way. And you’d be wrong.

When I’m talking date processing, I mean simple calendar date (not time) operations, like: get today’s date, add a number of days, compute the number of days between two dates, etc. The original sin, so to speak, is that Java doesn’t have a class to represent a day; Java’s Date class is actually more like a timestamp: it encapsulates a moment in time (with a millisecond precision) stored as the number of milliseconds to/from the moment called “epoch”, which is arbitrarily fixed at January 1, 1970, 00:00:00 GMT.

The methods related to date components (day of month, month, year, etc.) have all been deprecated since the apparition of the Calendar class (JDK 1.1). The trouble is, Calendar isn’t available in GWT’s JRE emulation library. So what do most programmers do? They use the deprecated methods of class Date. For example to create a Date that corresponds to a given day, they would use new Date(y, m, d). Well it’s deprecated, but it works, right?

Wrong. When instantiating a Date object using these deprecated methods, each JavaScript implementation tries to do its best to guess the correct timestamp, based on things such as the current time zone and whether Daylight Savings Time was effective at that time, and of course these rules vary by OS, JavaScript implementation and depend on the client’s regional settings. I’m not going into details now, but the bottom line is: you can’t expect to get a consistent behaviour.

Actually, if you might not notice this problem if all you do is manipulate local dates that were created on the client-side; the problem really becomes a nightmare when you mix locally instanciated dates with serialized dates from the server.

We’ve spent an incredible amount of energy and time to work around this issue, which is made worse by GXT’s DateWrapper and DateTextField. The awkward but kind-of working solution that we use now is to subtract Date.getTimezoneOffset() from Date.getTime() in order to “cancel out” the client’s time zone and DST. Not pretty.

If I had to do my current project again, I would definitely stay away from the Date type on the client side. As primitive as it may sound, if I have to manipulate dates without a time part, I would stick to using a (day, month, year) triplet and do as little date calculations on the client-side as possible.

You’ve been warned!