calendar by week

[GuineaPig]

Hello gurus,

Does anybody know of a function/module that can help me with the 
following problem:

In my app the user can encode the starting and ending hours of his 
workdays.  The app calculates the time the user worked (per week) and 
the user's overtime.  The results are exported to an excel-sheet for 
printing.

One thing that I would like to accomplish is that the user has to only 
give in the weeknumber of the year.  The app should then write the dates 
in this week to the excel-sheet.  I've been looking at the calendar and 
time modules but I don't quite see a solution there (maybe there is 
though, maybe I'm just too much of a newbie.)

Tnx a lot!

Tom