If there should only be one unique Broadcast date per episode (as opposed to multiples of rehearsals and repeats), then you should be able to use Max(EpisodeDates::BroadcastDate), where BroadcastDate is your calculation field in the Dates table: if( dates:datetype = "Broadcast" ; Dates:Date; "")
p.s. I would use GetAsDate("") for empty date calculations.
Thank you David, unfortunately the Max calculation will not work, as it will pick up repeats!
Just had a brain wave, if I set up a new field in Dates, that populates only when the type is "Broadcast", then I can have that populate with the date and use the Get Max calculation in the other field and it works!
Seems a bit messy, but it works. Is there a more streamlined method?