In the past I've stored dates-with-accuracy as a start date and an end date. The day may21,2012 would be represented as start=12am,may21,2012 and end=12am,may22,2012. The year 2012 would be represented as start=12am,Jan1,2012 end=12am,Jan1,2013.

I'm not sure if I'd recommend this approach. When displaying the information to the user you need to properly detect that a date range exactly covers a day in order to show "may 25" instead of two over-specific endpoints (which means dealing with daylight savings and so forth).

However, when you're not trying to translate to human, programming with the endpoints is a lot easier than with center+accuracy. You don't end up with lots of cases. That's pretty nice.