Try:
IF {Data.ApptDte} = datetime(1800,1,1,0,0,0) then
datetime(0,0,0,0,0,0) ELSE
{Data.ApptDte}
This would return a blank field--is that your goal? The error message is telling you that ApptDte is a datetime and therefore must be compared to a datetime, not a string.
-LB