Java calculating past date from today is going into the future

Question

I'm having an issue in Java calculating the current date minus a certain amount of days.

I have:

Date pastDate = new Date(new Date().getTime() - (1000 * 60 * 60 * 24 * 25));

This is returning Tue Feb 16 09:04:18 EST 2016 when it should actually return Tue Dec 28 16:06:11 EST 2015 (25 days into the past).

What's very strange is that for any number under 25 days works completely fine:

Date pastDate = new Date(new Date().getTime() - (1000 * 60 * 60 * 24 * 24));

As in 24 days into the past returns a predictable Tue Dec 29 16:06:11 EST 2015.

Any help would be appreciated.

Answer 1

With 24 days, the product remains just below the maximum possible int value, Integer.MAX_VALUE, which is 2,147,483,647. The 24 days product is 2,073,600,000. The 25 days product is 2,160,000,000. The result is overflow and a negative number, resulting in a date in the future.

For such values, use a long literal for the first value (or cast it to a long) to avoid the overflow that comes with exceeding Integer.MAX_VALUE. Note the L appended to 1000L:

(1000L * 60 * 60 * 24 * 25)

This works fine because the desired constructor for Date takes a long.

Date arithmetic is handled more cleanly by using Calendars, where you can explicitly add a negative number of days.

Also, with Java 8+, you can use Instant and its minus method to subtract the time.

Instant.now().minus(24, ChronoUnit.DAYS);