time date math problem

[Felix Seeger]

Hi

What I want:

Date:

20.03.2002

20.05.2002 - 21.00.0000 = 30.04.2002


I need this for displaying an archive of files by date:
The last files from this week
Than ever week a file
After one month every month a file
and so on.


How can I do this ?



thanks
have fun
Felix