This is the recommended expression for rounding dates to the end of the current plan year:

EOY_DT:= #ENDMTH (2 #LSTBUSDAY A_DT) &
; find the last business day of the current plan year, then round that last business day
; to the end of the month (i.e., last day of the plan year)

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 then we need to use #ENDMTH to guarantee that we are at the end of the current plan year. 2 #LSTBUSDAY A_DT finds the last business day of the current plan year, which is one of the last 3 days of the current plan year, depending on whether the last day of the current plan year is a weekday, Sunday or Saturday. Then, #ENDMTH converts it to the last day of the month (i.e., the last day of the plan year).

​If you have already have calculated the beginning of the next plan year for each date in A_DT (e.g., stored in BOY1_DT), then simply subtracting 1 day gives you the end of the current plan year:

EOY_DT:= BOY1_DT - 1 &