hi Ian.
I wrote the following formula. I think it will work.
if currenttime in time (19,00,00) to time (23,59,59) then {@scan_date_time} >= datetime (today, time(19,00,00)) else if
currenttime in time (00,00,01) to time (03,00,00) then {@scan_date_time} >= datetime (today -1, time (19,00,00))