I would call your column something other than DATE - it's not a function by itself but it's pretty close to being a reserved word. Suppose your input datetime string is in a column called TEXT DATE. The following formula will do the trick:
dt << New Column( "Real Date", Numeric, Continuous,
Format( "ddMonyyyy", 10 ), Input Format( "ddMonyyyy" ),
Formula( if (!isempty(:Text Date),
Informat(Substr( :Text Date, 1, 8 ), "ddMonyyyy")
)
)
);