How dose Postgresql store date internaly?

Hi,

can one tell me how Postgresql 7.3 stores the type DATE internally. I
know, I can set the datestyle to get the output in various formats.
But I would like to know, how a date is sored by the backend.
Maybe in milliseconds?

I think it depends on the datestyle how the date is interpreted for
inserts. So, dose the datestyle effect both, input and output of
temporal (except interval) data?

Thanks Erwin