It probably has something to do with where to point the thing during re-entry. The shuttle would need to be able to land on different days in case something went wrong. Actually, the date stuff is probably used for all sorts of orbital calculations.

It doesn't - but can you guarantee that everything it is talking to doesn't also change?

A GPS string might erroneously say it's day 367 or be one character too long because it printed 20010 or a telemetry link might ask for 4million readings because it calculated "-1"