
Explanation

Explanation:
Box 1: A DAX Calculated measure
Box 2: Date.EndofQuarter(#date([Year],[Mont],1))
ENDOFQUARTER returns the last date of the quarter in the current context for the specified column of dates.
The following sample formula creates a measure that returns the end of the quarter, for the current context.
= ENDOFQUARTER(DateTime[DateKey])
Reference:
https://docs.microsoft.com/en-us/dax/endofquarter-function-dax