Calculate the difference between two dates in Java

Avoid the pitfall of between and until

There are some use cases that require to count the number of days between two dates.
For example you need to count the days between the 1st of January 2019 and the 31th of December 2019.

LocalDate offers the method until that is equivalent to
TemporalUnit.between(Temporal, Temporal)

The pitfall of these methods is that the first temporal object is inclusive and the second temporal object is exclusive.

In the first test you can see that between January 1st and December 31 there are 364 days because interval between the 2 dates doesn't include 365 full days.

@Test 
public static void daysBetween() { 
  LocalDate firstDayOfTheYear = LocalDate.of(2019,1, 1); 
  LocalDate lastDayOfTheYear = LocalDate.of(2019,12,31); 
 
  // between Jan 01 2019 and Dec 31 2019 there are 364 days 
  assertEquals(364, ChronoUnit.DAYS.between(firstDayOfTheYear, lastDayOfTheYear)); 
  // between Jan 01 2019 at 00:00 and Dec 31 2019 at 23.59999 there are 364 days 
  assertEquals(364, ChronoUnit.DAYS.between(firstDayOfTheYear.atStartOfDay(), lastDayOfTheYear.atTime(LocalTime.MAX))); 
  // between Jan 01 2019 at 15:00 and Dec 31 2019 at 14:59 there are 363 days 
  assertEquals(363, ChronoUnit.DAYS.between(firstDayOfTheYear.atTime(15,00), lastDayOfTheYear.atTime(14, 59))); 
} 
 

In our second test we show that between January 1 midnight and December 31 at 23:59:59 we have 364 days.

In our third test we see that if the time of the begin date is later than the time of the end date, we could get 363 days as result.
363 is the total of full days between the start and the end.

How to count the days

Java doesn't offer the possibility to calculate the number of days using two inclusive dates.

If you want to count the number of days between two dates including them, you can use between or until
but you have to add 1 to the result or add 1 day to your end date:

 assertEquals(365, ChronoUnit.DAYS.between(firstDayOfTheYear, lastDayOfTheYear) + 1); 

 assertEquals(365, DAYS.between(firstDayOfTheYear, lastDayOfTheYear.plus(1, DAYS)));