Best date format for timestamps in a MongoDB document?

MongoDB has a built-in datetime type which interacts with the datetime types in your application's language (in PHP they become MongoDate instances). MongoDB datetimes are always stored in UTC, and are internally stored as milliseconds since the UNIX epoch. This means that they are compact (they are always 8 bytes, as opposed to string formats which are longer depending on how much precision you choose to store). Additionally, the MongoDB tools all "understand" datetime objects -- you can manipulate them easily from javascript in a Map-Reduce, or using the new aggregation framework.

Over the years, I have been forced to a very strong personal commitment to always storing date/time as the 10-digit Linux/Unix timestamp, which gives the current (add: local) time as seconds since the Epoch. Just a few moments ago, the time was 1329126719. To me, this is the most flexible format possible. When it comes time to display a date/time, it's simple to convert the 10-digit timestamp to any string you care to show.

Edit: Perhaps a better choice for me would be milliseconds from the Epoch, since that seems to be increasingly favored as the art evolves.