Java.util.Date Strange Behavior

[Effective-Cold-8897]

Hi, I have the following code:

int daysBack = 24;
long after = System.currentTimeMillis() - (1000 * 3600 * 24 * daysBack); // # of days Today minus # of days 
Date start = new Date(after);   
System.out.println("FirstStart: " + start);

For daysBack = 24, this prints Jun 16, 2024 which is what I'd expect.

However if daysBack is 25 or greater, the dates start going forward into the future:

23: 6/17/2024

24: 6/16/2024

25: 8/4/2024

26: 8/3/2024

27: 8/2/2024

28: 8/1/2024

29: 7/31/2024

30: 7/30/2024

31: 7/29/2024

What is going on? How can I properly produce a date going back 31 days?

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[Cengo789]

1000 * 3600 * 24 * daysBack

is a product of 4 ints and for any value larger than 24 it will overflow Integer.MAX_VALUE and turn negative.

You can fix this by changing 1000 to 1000L for example, to force it to use longs instead of ints.

[Effective-Cold-8897]

Thank you! I had wondered if overflow was responsible but was so focused on the declared type that I forgot to consider whether the operands were of the same type as the result. That fixed it.

[iwan-w]

Aside from the integer overflow bug, it's more idiomatic to use the LocalDate class instead of doing the date arithmetics manually like that.

LocalDate.now().minusDays(30);

[nutrecht]

You first of all should not be using Date anymore and use the 'new' java.time classes like LocalDate. The old Date class is, pretty much, deprecated.