计算包括两个给定日期的天数

Question

How to count the number of days between two LocalDate 's in java? I tried like this

LocalDate startDate = LocalDate.parse("2016-11-02");
LocalDate endDate = LocalDate.parse("2016-11-04");

long days = startDate.until(endDate, ChronoUnit.DAYS);

//and

long days= ChronoUnit.DAYS.between(startDate, endDate);

//and

long days= Duration.between(startDate.atStartOfDay(), endDate.atStartOfDay()).toDays();

All above are giving days between two dates, but how to count inclusive of both dates. As per above steps, getting 2 days as no. of days, but required 3 days.

Answer 1

The javadoc is clear that the end date is not included in the calculation. So you can simply add 1:

long days = ChronoUnit.DAYS.between(startDate, endDate) + 1;

Answer 2

If you want literal 24 hour days, you can use the Duration class instead:

long days = Duration.between(startDate.atStartOfDay(), endDate.atStartOfDay()).toDays() + 1;

For more information, please see this document regarding Java SE 8 Date and Time.

Answer 3

tl;dr

Beginning being inclusive , and ending being exclusive in a span of time is the norm.

If you absolutely cannot follow that approach, add one.

days = days + 1 ;

Half-Open

A common and wise way to define a span of time is the Half-Open approach. The beginning is inclusive while the ending is exclusive .

Sometimes, but not always, we use this approach intuitively in everyday life. For example, a classroom's lunch period is said to be noon to 1 PM which means all the students are to be returned to class and ready before the class strikes 13:00. Class is said to run 1 PM to 2 PM. So the spans can abut one another without overlapping, and without the tricky task of determining the last moment of an infinitely divisible last second of last minute of that hour.

I suggest following this approach in all your date-time coding will make your code easier to read, easier to debug, and less error-prone by reducing the cognitive overload of resolving the inclusive-exclusive ambiguity.

The java.time classes use the Half-Open approach throughout.

long days = ChronoUnit.DAYS.between( start , stop ) ;  // Half-open

If you code must return an ending-inclusive result, just add one to the java.time results.

long daysClosed = ( days + 1 ) ;

Ideally you would use Half-Open consistently in your code and in your user interface, while educating your users as to the issue of ambiguity. I have seen countless mistakes made by businesspeople wrongly assuming that date ranges were open () , closed [] , or half-open [) , and even occasionally the opposite half closed (] while the author meant another.