The numbers you are doing the math with (150 * 1000 * 60 * 60 * 24) are all integers, which means the result will be an integer. Integers have a max value which this calculation exceeds, so it wraps around and becomes negative - and probably comes back up to being just a few hours ahead instead of a lot of days ahead. Since the result you need has to be a long, you should use longs in the calculation - at least one of the constants should be a long so the rest of them can be widened:
Or make it a little more readable:
Also, take a look at the Calendar class, it has methods built in for doing date math, which you should prefer to use.