Please clarify what you have, you cannot have a "month" in a date9. field. A date variable holds a number which is reflective of the number of days since the cutoff period. This then has a format applied to it like date9 which displays the number of days in a readable format.
Next up "Year <= Month" - this does not make any sense as only years up to 12ad will be <= month which can only be 1-12. So please clarify what you want, provide some example data.