Help with date math

I need some help. I am trying to replicate a function from Sybase ASA, and
am having difficulty.

I need to be able to subtract 2 date (or timestamps) and return the results
expressed in days, weeks, month, quarters, or years. How do I do this?

I believe Postgres is returning the number of days when you subtract to
days.
i.e. postgres=# select current_date - '2007/01/01';
?column?
----------
200
(1 row)

However, I can not figure out how to get the results expressed as 7 months,
or 2 quarters, x weeks, or 0 years. I have tried to use date_part, but
since the subtraction is returning an integer, it is not happy.

Any help would be greatly appreciated.

Chris

P.S.

I'm trying to replicate Sybase's datediff function.