As far as a datepicker, if your budget allows it look at the date and time controls at http://www.peterblum.com/DES/DateAndTime.aspx. They are very well designed and probably a lot cheaper ($90 for the package) than the cost to develop your own..

Actually with a "consultants license" the cost is $0.
It costs money as soon as you place it on one of your customers' production servers.

Peter Blum's stuff looks awesome.

Dave.