i think i figured it out
input startDateYyyyMmDd = 20181118;
input startTimeHHMM = 0930;
def beyondStartDate = if GetYYYYMMDD() >= startDateYyyyMmDd then 1 else 0;
def beyondStartTime = if SecondsfromTime(startTimeHHMM) >= 0 then 1 else 0;
plot VWAP = TotalSum(if beyondStartDate and...