The basic principle was a calendar with all possible dates linking to the main fact table and flags / pointers describing the type of link:
This gives endless possibilities when creating time comparison calculations. Since then I've developed the idea further with additional functionality added:
Comparison of Special Dates which can move year on year (For example Easter)
Two new Fields; From Date and To Date which are associated to dates which fall in-between
Financial Month, Qtrs, Years added
Rolling averages and accumulation felds added from HIC's recent blog post: The As-Of Table
I'll be talking through how this calendar works soon on my blog www.qlikcentral.com . I also have developed a technique where expressions and nested expressions are replicated using the flags from the calendar so you don't have to manually set every possibility up!