HPDATEVALIDATE is an HP intrinsic, not (so far as I know) a Cobol function.
The basic form is:
CALL INTRINSIC "HPDATEVALIDATE"
USING DATECODE, INPUTDATE, CUTOFF
GIVING RESULT.
See http://docs.hp.com/dynaweb/smpe/b1019/556/@Generic__BookView
for more details.