A TimeBomb is bug that will cause code to eventually fail but that works at the moment. Of course, the various Y2K problems fell into this category, but it includes lots of other, similar things. One example is a unique key generator that fails to properly handle arithmetic overflow (often because of sign extension problems).
Another example is generating the date of "one month from now" by changing the month field on a date structure. Thus, the date constructor upchucks when being given the date of "February 30th, 2005" on January 30th, 2005.