M day, M Day, and M-day redirect here; for the Marvel Universe event, see Decimation (comics). H-Hour redirects here; for the D-Day designation, see H-hour (D-day).

The military designation of days and hours within the North Atlantic Treaty Organisation (NATO), is specified in AAP-6 (STANAG 3680), NATO Glossary of Terms and Definitions, and marked (NATO) in what follows. Those entries marked (US) are specific to the U.S., and defined only in Joint Publication JP 1-02, Department of Defense Dictionary of Military and Associated Terms]