Hi there.
First of all, you should read this: https://dax.guide/firstdate/ and when you do, please read it WELL with full attention to the DETAILS.
Secondly, you should be aware that FIRSTDATE is a table function that returns a TABLE, not a value (even though it can be automatically converted into a value on demand).
Thirdly, you should use this function only on A PROPER DATE TABLE. Same is (even more) true for DATEADD. If you don't follow the rules, then you'll be sorry rather sooner than later 🙂 Please mark my words well.
FIRSTDATE returns the very first date in the column passed in as the parameter. DATEADD shifts a column by the desired amount of time BUT ONLY IF the shifted period is present in the passed-in column. If not, then BLANK will be returned and hence all other time-related functions will return BLANK. Let me warn you again: DO NOT USE TIME-INTELLIGENCE FUNCTIONS ON NON-DATE TABLES.
Best
Darek
