Calculating the difference between two Java date instances

Question

I'm using Java's java.util.Date class in Scala and want to compare a Date object and the current time. I know I can calculate the delta by using getTime():

(new java.util.Date()).getTime() - oldDate.getTime()

However, this just leaves me with a long representing milliseconds. Is there any simpler, nicer way to get a time delta?

Answer 1

Simple diff (without lib)

/**
 * Get a diff between two dates
 * @param date1 the oldest date
 * @param date2 the newest date
 * @param timeUnit the unit in which you want the diff
 * @return the diff value, in the provided unit
 */
public static long getDateDiff(Date date1, Date date2, TimeUnit timeUnit) {
    long diffInMillies = date2.getTime() - date1.getTime();
    return timeUnit.convert(diffInMillies,TimeUnit.MILLISECONDS);
}

And then can you call:

getDateDiff(date1,date2,TimeUnit.MINUTES);

to get the diff of the 2 dates in minutes unit.

TimeUnit is java.util.concurrent.TimeUnit, a standard Java enum going from nanos to days.

---

Human readable diff (without lib)

public static Map<TimeUnit,Long> computeDiff(Date date1, Date date2) {

    long diffInMillies = date2.getTime() - date1.getTime();

    //create the list
    List<TimeUnit> units = new ArrayList<TimeUnit>(EnumSet.allOf(TimeUnit.class));
    Collections.reverse(units);

    //create the result map of TimeUnit and difference
    Map<TimeUnit,Long> result = new LinkedHashMap<TimeUnit,Long>();
    long milliesRest = diffInMillies;

    for ( TimeUnit unit : units ) {

        //calculate difference in millisecond 
        long diff = unit.convert(milliesRest,TimeUnit.MILLISECONDS);
        long diffInMilliesForUnit = unit.toMillis(diff);
        milliesRest = milliesRest - diffInMilliesForUnit;

        //put the result in the map
        result.put(unit,diff);
    }

    return result;
}

http://ideone.com/5dXeu6

The output is something like Map:{DAYS=1, HOURS=3, MINUTES=46, SECONDS=40, MILLISECONDS=0, MICROSECONDS=0, NANOSECONDS=0}, with the units ordered.

You just have to convert that map to an user-friendly string.

---

Warning

The above code snippets compute a simple diff between 2 instants. It can cause problems during a daylight saving switch, like explained in this post. This means if you compute the diff between dates with no time you may have a missing day/hour.

In my opinion the date diff is kind of subjective, especially on days. You may:

• count the number of 24h elapsed time: day+1 - day = 1 day = 24h

• count the number of elapsed time, taking care of daylight savings: day+1 - day = 1 = 24h (but using midnight time and daylight savings it could be 0 day and 23h)

• count the number of day switches, which means day+1 1pm - day 11am = 1 day, even if the elapsed time is just 2h (or 1h if there is a daylight saving :p)

My answer is valid if your definition of date diff on days match the 1st case

With JodaTime

If you are using JodaTime you can get the diff for 2 instants (millies backed ReadableInstant) dates with:

Interval interval = new Interval(oldInstant, new Instant());

But you can also get the diff for Local dates/times:

// returns 4 because of the leap year of 366 days
new Period(LocalDate.now(), LocalDate.now().plusDays(365*5), PeriodType.years()).getYears() 

// this time it returns 5
new Period(LocalDate.now(), LocalDate.now().plusDays(365*5+1), PeriodType.years()).getYears() 

// And you can also use these static methods
Years.yearsBetween(LocalDate.now(), LocalDate.now().plusDays(365*5)).getYears()