[Jprogramming] two calendar questions

[david alis]

I need two conversion verbs:
   (year,day_number) into (year,week_number)
for example - (2010,1) -> (2009,53)
(1.Jan.2010 is in Week 53 of 2009)

and the simpler:
 (year, day_number) into (year, month_number)

Are there any scripts that I should look at?

Thanks
David