Calculating rolling 52 week values from annual/interim reports

If the last report released by a company was the full year report (a) then obviously the rolling 52 weeks comes directly from there, however if the most recent report was an interim one (b) then would you also consider the previous interim report (c) to calculate the rolling 52 week period?

eg:
b) 01/01/2008 (Interim)
a) 01/07/2008 (Full Year)
c) 01/01/2009 (Interim)

Rolling 52 weeks = a - b + c

Or is there a simpler way?