Returns the last date for the given month, quarter or year.
openingBalanceMonth = OPENINGBALANCEMONTH(SUM(TransactionData[Revenue]),CalenderDate[Date])
openingBalanceQuarter = OPENINGBALANCEQUARTER(SUM(TransactionData[Revenue]),CalenderDate[Date])
openingBalanceYear = OPENINGBALANCEYEAR(SUM(TransactionData[Revenue]),CalenderDate[Date])
Opening balance month for February displays the last day of revenue in January.