I agree client-side is the way - your users wd get bored schlepping back and forth to the server to get new dates. Try
(n.b. it'll give you some jscript errors because in implementation it sends the date to another window, which in this case you have not got).
I hasten to add that I didn't write the basic jscript for this - I cannibalised it with permission from an entry in the 5kb website contest a few yrs ago by Martin Kastel - I think it's tip top)