I had to put a @ClientType in my field formula to get around this, but it seems pretty odd that the same data needs to be formatted differently to work in the two clients. But now it’s suddenly decided that text is ok for both (??!) so I guess it’s a moot point.
temp:=@If(@ClientType=“Web”; @Weekday(RoomDate_d); @Weekday(@TextToTime(RoomDate_d)));