I suggest a measure on either DateDim or FactTable:
DateLastFact:=calculate(max(FactTable[TransactionDate]),all(FactTable))
Then a calculated column in DateDim:
Within365Flag=if([date]>[DateLastFact]-365 && [date]<=[DateLastFact],"Y",blank())
I like having [DateLastFact] as a separate measure so it can be surfaced somewhere on the dashboard for the users to know the age of the dataset.