Question

I want to perform a date operation in my android app. What I want is to subtract two dates and get the result. But subtraction leads to the wrong result whenever I change the time zone to central daylight time.

I use the following code to find the difference between the two dates.

Long lDateDiff = dtCycleDay.getTime() - m_dtHistory[0].getTime();
lDateDiff = lDateDiff / (1000 * 60 * 60 * 24);

Here in m_dtHistory[0], the date stored is Thu Mar 01 00:00:00 CST 2012.

And in my dtCycleDay variable the date changes from Thu Mar 01 00:00:00 CST 2012, Thu Mar 02 00:00:00 CST 2012, Thu Mar 03 00:00:00 CST 2012… and so on.

Now up to Thu Mar 11 00:00:00 CST 2012, the subtraction result is fine, but when the date changes to Thu Mar 12 00:00:00 CDT 2012, the CST changes to CDT and it show wrong subtraction result.

Why this happens and these happen only when I change the time zone to Central Daylight Time or pacific Daylight Time.

Answer 1

What do you mean by the “wrong” subtraction result?

My guess is that the result is 23 hours or 25 hours- which is exactly what I’d expect when a daylight transition occurs, as the intervening day is longer or shorter in terms of elapsed time. The “longer” day won’t be relevant when dividing by 24, but the shorter one will… you’re assuming that every day has 24 hours, and that you can therefore count the number of days by dividing the elapsed milliseconds by “the number of milliseconds in 24 hours”. That doesn’t work due to varying day lengths.

Don’t forget that a Date value is purely an instant in time. It doesn’t know about calendars or time zones… if you want to know the difference in “local” dates and times (where midnight to midnight is always 24 hours), I’d suggest using Joda Time instead… Date and Calendar don’t really do that for you.

If the real problem you’re describing is the time zone changing at the wrong date, that’s a different matter entirely, and could be due to various different causes. For one thing, you should show exactly which time zone you’re talking about: the abbreviations are ambiguous, whereas the tzdb names (e.g. “Europe/Paris”) aren’t.