In a recent post to his site Ross Tuck looks at improving time objects with imprecision, basically determining the granularity you require and using that in all time operations.
When modeling important numbers, it’s considered good form to specify the precision. Whether it’s money, size or weight; you’ll typically round off to a given decimal point. Even if it’s only for user display, rounding off makes the data more predictable for manipulation and storage.
Unfortunately, we don’t often do this when handling time and it bites us in the rear.
He gives an example of a use of the DateTime handling in PHP and possible inconsistencies when working with the time portion. He offers three possible solutions: one where the time is specified correctly, making a relative modification on the object being checked and, his ultimate solution, just "throwing away" the time and not worrying about it. He then shows how to encapsulate this checking and a few other issues that can come with date/time and evaluations (like equals checks).