Your "Grace Period - Principle (M)" is probably DateFormula type and adding M will be potentially bad.
1. Futuredate:=CALCDATE(FORMAT("Grace Period - Principle (M)"+'CM',PostingDate);
2. Better is to fill your "Grace Period - Principle (M)" field with value which can be directly usable
3. Use <> around dateformula string inside CALCDATE function to avoid language influence.
Answers
This
Futuredate:=CALCDATE('CM',PostingDate);
Futuredate:=CALCDATE(StringDate,PostingDate);
1. Futuredate:=CALCDATE(FORMAT("Grace Period - Principle (M)"+'CM',PostingDate);
2. Better is to fill your "Grace Period - Principle (M)" field with value which can be directly usable
3. Use <> around dateformula string inside CALCDATE function to avoid language influence.
"Grace Period - Principle (M)" its Integer;
I worked with it as integer.