What's going on with Java's Date class? Is this a known bug?

Question

I know Date is mostly deprecated, but I still use it form time to time (less code than using Calendar). I came across a truly bizarre bug, and I'm wondering if anyone can explain this to me.

This code, which adds 24 days to the current time:

long nowL = System.currentTimeMillis();
Date now = new Date(nowL);
System.out.println("now = "+now);
Date future = new Date(nowL+ 24*24*60*60*1000);
System.out.println("future = "+future);

gives this correct output:

now = Thu Jun 11 10:50:09 IDT 2009

future = Sun Jul 05 10:50:09 IDT 2009

while this code, which adds 25 days:

long nowL = System.currentTimeMillis();
Date now = new Date(nowL);
System.out.println("now = "+now);
Date future = new Date(nowL+ 25*24*60*60*1000);
System.out.println("future = "+future);

gives this output:

now = Thu Jun 11 10:51:25 IDT 2009

future = Sun May 17 17:48:37 IDT 2009

I can understand a difference of hours, even days, but can anyone explain why adding too many milliseconds causes going back in time?? I'm baffled.

Answer 1

25*24*60*60*1000 = 2160000000 = 0x80BEFC00

you are computing an integer value, and get an overflow. if it was

25*24*60*60*1000L

everything should be fine.

Answer 2

This isn't a bug in the Date class, it's a case of integer overflow. ints in Java can only be between -2³¹ and 2³¹ - 1, but 25 × 24 × 60 × 60 × 1000 is greater than 2³¹ - 1 so it overflows.

If you run

System.out.println(24*24*60*60*1000);
System.out.println(25*24*60*60*1000);

you get the results

2073600000
-2134967296

If you specify one of the numbers you're multiplying together as a long by adding the L suffix to it, the product will also be a long. long values can go up to 2⁶³ - 1 so you won't get overflow unless you're adding a lot of days to your Dates. For example,

System.out.println(25L*24*60*60*1000);

gives you

2160000000