I'm trying to create two named sets.  The first is a rolling 13 months, the second is the sum of those 13 rolling months.  LastPeriods() with .CurrentMembers doesn't appear to work as the result pulls all members.  If I use an explicit member the LastPeriods() function works, but does not roll.  My code is below.  Any suggestions are greatly appreciated!

LastPeriods(13,[Time].[Default].&[2017P02])         'Works but doesn't roll
LastPeriods(13,[Time].[Default].CurrentMember)    'Pulls all members rather than the last 13?
SUM(LastPeriods(13,[Time].[Default].&[2017P02]),[Measures].[Value])  'Error - "Invalid Named Set Definition: The Axis0 function expects a tuple set expression for the argument. A string or numeric expression was used."