In XSLT 2.0, define a key to give direct access to schedule entries, where the key value is the computed timeslot for each event.
If you're stuck with 1.0 it's not so easy to define a key on a computed value. Try a two-pass solution: in the first pass, attach an attribute to each entry indicating the timeslot of each item in the schedule; in the second pass, define a key on this attribute.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference
XSLT variable help needed
