I tried that, all of that. I will attempt to put the solution in words.

1. Calculate the dateDiff in months between the Start and End Dates

S: E: Diff:

2008-06-15 2008-08-10 2

2. Join to a Table (tally, derived, CTE ) of numbers that covers your range including 0

Start End Diff Num

2008-06-15 2008-08-10 2 0

2008-06-15 2008-08-10 2 1

2008-06-15 2008-08-10 2 2

3. Calculate the Start of months (and therefore the End )

Start End Diff Num NewStart NewEnd

2008-06-15 2008-08-10 2 0 2008-06-01 2008-06-30

2008-06-15 2008-08-10 2 1 2008-07-01 2008-07-31

2008-06-15 2008-08-10 2 2 2008-08-01 2008-08-31

NewStart = DateAdd(Month, DateDiff( month, 0, Start) +Num ,0 )

Then do the Case When NewStart > Start Etc.