The rating is valid to the date specified on your ratings page. If it is valid to the end of the month, rather than the anniversary of the test, that will be reflected on the expiry date written down
JAR–FCL 1.315Instructor ratings and
authorisations – Period of
validity
(a) All instructor ratings and
authorisations are valid for a period of 3 years.
(b) The validity period for a specific
authorisation shall not exceed 3 years.