Actually, I didn't change anything.

I narrowed it down to a function that subtracted two timestamps, but that doesn't explain why it failed.
I've removed the function, as a workaround. Frankly, I can't see what fault it finds with it.

Time goes forward, so the numbers get larger. As Forger suggested, it's probably something to do with the bitdepth of the variables, however, php is designed to sort that out behind the scenes, so I suspect it's their fault