There's plenty of capability in XSLT 2.0 for date/duration arithmetic. But there's not much in 1.0. If you're stuck with 1.0, see if your XSLT processor supports the date/time library functions defined at
www.exslt.org
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference