INTDATE(ANSI) instructs the compiler to use the Standard COBOL 85 starting date for integer dates used with date intrinsic functions.
Day 1 is Jan 1, 1601. INTDATE(LILIAN) instructs the compiler to use the Language Environment Lilian starting date for integer dates used with date intrinsic functions.
Day 1 is Oct 15, 1582.

INTDATE option syntax

Default is: INTDATE(ANSI)

Abbreviations are: None

With INTDATE(LILIAN), the date intrinsic functions return results that are compatible with the Language Environment date callable services.

Usage note:
When INTDATE(LILIAN) is in effect, CEECBLDY is not usable because you have no way to turn an ANSI integer into a meaningful date by using either intrinsic functions or callable services.
If you code a CALL literal statement with CEECBLDY as the target of the call when INTDATE(LILIAN) in effect, the compiler diagnoses this and converts the call target to CEEDAYS.