I'm not sure if this will work for your setup,
but maybe it can give an idea.
It employs the technique described here:
Cross joining against a list of all month starts and ends,
then filtering using:
[Date Creation (Months)]<=[Lookup Start (Months)]
AND [Solve Date (Months)]>=[Lookup Start (Months)]
Then Ticket Age in a given month is:
DATEDIFF('day',[Date Creation],[Lookup End])
and gets flagged if over 30:
IF [Ticket Age in a Month]>30
THEN 1 ELSE 0 END
This flag can be summed to get the count per month.
Please see workbook attached in Forum Thread.
Here are other links on the subject:
Thank a lot for your answer Swaroop, I'm trying this out.