When using the prior period function and you want the calculation to work on any level of the hierarchy, you'll need to remember the "CurrentMember.Level" syntax:
CREATE MEMBER CURRENTCUBE.[Measures].[View Count - Prior Period]
[Dim Date].[Calendar Year Hierarchy].CurrentMember.Level
, [Dim Date].[Calendar Year Hierarchy].CurrentMember
, [Measures].[View Count]
This will allow the calculation to show Prior Year when looking at the year level, Prior Month when looking at the month level, etc.