Detecting ALL() - Detecting When Not Filtered

File this under "you may never need this, but when you do, you'll know it immediately." I'd call this one a solid 4 on the DAX Spicy Scale.

Why I needed this: I had two different calendar tables, one at the Date level and one that was a custom calendar, Periods table. Most of the measures in this model are written to be used with one table or the other, and I never have to "cross the streams."