JavaScript: Creating timestamps with time zone offsets

I was converting the example code of my Windows Azure session to Visual Studio 2010 solution called MVC Time Planner when I discovered some date and time offset issues in my JavaScript code. In this posting I will show you some useful tricks you can use to convert JavaScript dates and times to timestamps.

Date.getTime() returns UTC

When you call getTime method on Date object you get the number of milliseconds from Unix epoch. Although your current Date object keeps time with some offset getTime gives seconds in UTC. Keep this in mind when creating timestamps if you are not living on zero-meridian.

var currentDate = selectedDate;          // current date with offset
var currentTime = currentDate.getTime(); // offset is ignored

This is pretty awkward, unexpected and unintuitive behavior but you have to keep in mind that all date and time calculations must use same time system to give appropriate results.

Date.getTimezoneOffset()

getTimezoneOffset method returns you the amount of minutes you have to add to your current timestamp to convert it to UTC time. If your time zone is UTC+3 then getTimezoneOffset returns you –180 because:

UTC + 3 hours + (-180) minutes = UTC

If your time zone is UTC-3 then

UTC – 3 hours + 180 minutes = UTC

Pretty simple math but confusing at first place.

Getting timestamp with time zone

In my application I have to give timestamp for selected dates and times to server. I need to find correct amount of seconds from Unix epoch so users can save times based on their region and not by UTC. Here is how calculate timestamps.

var currentDate = selectedDate;
var currentTime = currentDate.getTime();
var localOffset = (-1) * selectedDate.getTimezoneOffset() * 60000;
var stamp = Math.round(new Date(currentTime + localOffset).getTime() / 1000);

On server side I use the following method to convert timestamp to date. This method is borrowed from CodeClimber blog posting Convert a Unix timestamp to a .NET DateTime.

private static DateTime ConvertFromUnixTimestamp(double timestamp)
{
    var origin = new DateTime(1970, 1, 1, 0, 0, 0, 0);
    return origin.AddSeconds(timestamp);
}

Conclusion

Playing with dates and times is always interesting thing to do because there are many different time zones in the world and supporting them all is not easy thing to do. Inside the system we must be able to keep all dates in appropriate system and usually it is UTC. This posting showed you some tricks about how to play with local times in JavaScript.