According to http://www.cs.unc.edu/~dm/UNC/COMP205/LECTURES/ERROR/lec23/n..., the system stored the time in integers, but it was converted to floating point when doing the conversions. This conversion contained a small error that accumulated over time.
Keep in mind that things that seem very WTF to you, might seem more plausible when given more details about the subject.
Well I have to give them that this is kind of a curious bug. I mean, you have to wait over 4 days until it triggers. Probably, this just worked whenever it was booted and tested, and booted and certified, because probably the certification did not involve ignoring the thing for four full days.
According to a book I read about game testing it's common for commercial games to be run through a test of simply leaving the game on for hours or days to see if there are bugs that only show up in this way.
In part because the user manual included instructions to regularly reboot before precision became a problem. The users did not, because they didn't want to risk being in the middle of a reboot when a target went overhead.
