Date function not working as intended?

Perhaps use DATEADD instead?

https://msdn.microsoft.com/en-us/library/ee634905.aspx

Well, DATEADD should work just fine, I have never seen a bug with that functions, and it should be the way to go.

If, on the other hand, you want to do it manually, then you can follow a simple technique. Create a Y12M column in the Date table that contains (Year * 12 + Month). With that column in place, you can freely move back and forth with months by adding to that value the offset.

So, MAX ( Date[Y12M] - 3 ) results in three months ago, no matter which year you are into.

But, if I were you, I would try to understand what's wrong with DATEADD, as this is the best way of expressing date offsets.

Have fun with DAX!

Alberto Ferrari
http://www.sqlbi.com