Compare values from specific dates
I want to compare values from two specific dates (let's say 31st of March 2018 and 31st of December 2017, so exactly 3 months or a quarter).
The idea is to compare how the values changed between then and now. The same I would need for a comparison of 6 months (e.g. 31st of March 2018 and 30.09.2017)
My approach would be to calculate the value (sum of CTC) for each project for the day which is 3 months back (6 months back, respectively)
The 1 months comparison I can do with "Previous" but I want not to compare 31st of March with 28th of February but with 31st of December. I want to write the formula as "dynamic" as possible, meaning that in April the 3 month comparison must be with the values of the 31st of January etc.
Formula for 1 month comparison: Sum([CTC]) over (intersect([Project_ID],Previous([Date])))
I thought of manipulating the formula by doing something like this: Sum([CTC]) over (intersect([PSP-Element],Previous([Date], -3))), to get the 3 months comparison but it does not work.
Can you help me in resolving this?