I need your expertise here! please help.

I need a function that will convert a gregorian date to ISO date.
ex. 1/2/2002 will return 1-1-2002

ISO date start on the First week of the January with its day starts on 
monday 
it has 52 weeks and 7 days in 1 week.

ex2. april 7 2002 will return 15 - 7 - 2002

please help .... i need a function for this.... 

THANKZ.