This is the recommended expression for rounding dates to the end of the calendar year quarter:

EOQ_DT:= #ENDMTH (5 #LSTBUSDAY A_DT) &
; find the last business day of the current calendar year quarter, then round that last business day
; to the end of the month

A_DT may be either a single date (e.g., 7/16/2015, DOH, #DODEC) or an array of dates (e.g., #DATE). 

We start with #LSTBUSDAY because it's the easiest way to work with plan years, but can be generalized to calendar years. We then use #ENDMTH to guarantee that we are at the end of the calendar year quarter. 5 #LSTBUSDAY A_DT finds the last business day of the current calendar year quarter, which is one of the last 3 days of the current calendar year quarter, depending on whether the last day of the current calendar year quarter is a weekday, Sunday or Saturday. Then, #ENDMTH converts it to the last day of the month (i.e., the last day of the current calendar year quarter).