Question

There are two parts to this: first compute the start date. Then compute the elapsed time. For the second part, I found some good advice here: Calculate elapsed time in Java / Groovy

However, the first part is a problem for me. Here’s how it works:
The user indicates that the time span extends from an hour value (00 to 23). I do not have a starting date/time object of any sort – just an integer hour. From that I need to figure the start date (and then the elapsed time).

If the start hour is greater than the now hour, it was the prior day. In order to get an actual start date from that however, I need to potentially consider month and year boundaries as well as things like leap years and daylight savings time changes. Surely someone has solved a problem like this already. (I believe it can be quite complex.) Is there a proven solution that will let me compute how much time (in seconds) has actually elapsed from the given hour of the day (00 to 24) to now? (The start time will always be assumed to be on the hour.)

Answer 1

Firstly, I’d suggest using the Joda Time API. It’s the best date/time API available for Java, in my opinion.

Next you need to work out exactly what to do in various corner cases. In particular, suppose the user enters “1” and you’re near a daylight saving transition. It’s possible that 1am happened twice (if the the time went 1:58, 1:59, 1:00, 1:01 because of a transition back away from DST) or that it didn’t happen at all (if the time went 12:58, 12:59, 2:00 because of a transition forward into DST). You need to work out what to do in each of those situations – and bear in mind that this means knowing the time zone too.

Once you’ve worked that out, it may not be too hard. With Joda Time you can use withHourOfDay method to get from one time to another having set one component of the time – and likewise there are simple APIs for adding or subtracting a day, if you need to. You can then work out the time between two DateTime values very easily – again, Joda Time provides everything you need.

Here’s an example which doesn’t try to do anything with DST transitions, but it’s a good starting point:

import org.joda.time.*;

public class Test
{
    public static void main(String[] args)
    {
        // Defaults to current time and time zone
        DateTime now = new DateTime();
        int hour = Integer.parseInt(args[0]);

        DateTime then = now
            .withHourOfDay(hour)
            .withMinuteOfHour(0)
            .withSecondOfMinute(0);
        if (then.isAfter(now))
        {
            then = then.minusDays(1);
        }
        Period period = new Period(then, now, PeriodType.seconds());

        System.out.println("Difference in seconds: " + period.getSeconds());
    }
}