STARTOFMONTH DAX Function in Power BI
The OPENINGBALANCEMONTH DAX function returns a table containing a single column and single row with a date value, which specifies the first date of the month in the current context for the specified column of dates.
DAX Syntax STARTOFMONTH(dates)
The parameter dates specify a column that contains dates.
Example: Let’s create a measure with name "STARTOFMONTH Measure1".
DAX
 STARTOFMONTH Measure1 = STARTOFMONTH('Students List'[Dates]) 
The output of the above dax code is shown below:
