Andrew - great article; however, I have a question re using Custom Calendar in MDX. In my scenario, I have created a cutstom calendar with Year/Quarter/Month/Week. All works great and data is displayed fine - but when I use the MDX with ParallelPeriod (ParallelPeriod([D_TIME].[H_DATE].[Year],1,[D_TIME].[H_DATE].CurrentMember),[Measures].[Cost]) and my previous year has incomplete data (for instance Dec 2009 only). This MDX compares January 2010 vs December 2009 and I need it to compare December 2010 vs December 2009. Using server build dimension works just fine with this MDX. Are custom calendars limited in functionality when it comes to MDX?