If it is what I think it is, it'll be hard to test. Computers store time in a 32bit value which counts the number of milliseconds the computer has been on. This is used for timing in various applications (read: games). After about 7 weeks, this value overflows. It gets so big a 32bit value can't hold it anymore, so it wraps around to 0 again. At that point, the current time (0) is less than the previous time (something like 4294967295), making the difference between them negative. I think I can fix it, but it'll be hard to tell if the fix worked or not.