ce095d56_tes__Requirement_and_duration___the_end_of_the_last_day_of_the
(v) Age 40 or older a recreational pilot certificate, a private pilot certificate, a flight instructor certificate (when acting as pilot in command or a required pilot flight crewmember in operations other than glider or balloon), a student pilot certificate, or a sport pilot certificate (when not using a U.S. driver's license as medical qualification)
24th month after the month of the date of examination shown on the medical certificate.
Federal Aviation Regulation Sec. 61.23 - Medical certificates: Requirement and duration.
Then your medical certificate expires, for that operation, at the end of the last day of the