- (C-, D-, M-days end at 2400 hours Universal Time (Zulu time) and are assumed to be 24 hours long for planning.) The Chairman of the Joint Chiefs of Staff normally coordinates the proposed date with the commanders of the appropriate unified and specified commands, as well as any recommended changes to C-day. L-hour will be established per plan, crisis, or theater of operations and will apply to both air and surface movements. Normally, L-hour will be established to allow C-day to be a 24-hour day. a. C-day. The unnamed day on which a deployment operation commences or is to commence. The deployment may be movement of troops, cargo, weapon systems, or a combination of these elements using any or all types of transport. The letter “C” will be the only one used to denote the above. The highest command or headquarters responsible for coordinating the planning will specify the exact meaning of C-day within the aforementioned definition. The command or headquarters directly responsible for the execution of the operation, if other than the one coordinating the planning, will do so in light of the meaning specified by the highest command or headquarters coordinating the planning. b. D-day. The unnamed day on which a particular operation commences or is to commence. c. F-hour. The effective time of announcement by the Secretary of Defense to the Military Departments of a decision to mobilize Reserve units. d. H-hour. The specific hour on D-day at which a particular operation commences. e. H-hour (amphibious operations). For amphibious operations, the time the first assault elements are scheduled to touch down on the beach, or a landing zone, and in some cases the commencement of countermine breaching operations. f. L-hour. The specific hour on C-day at which a deployment operation commences or is to commence. g. L-hour (amphibious operations). In amphibious operations, the time at which the first helicopter of the helicopter-borne assault wave touches down in the landing zone. h. M-day. The term used to designate the unnamed day on which full mobilization commences or is due to commence. i. N-day. The unnamed day an active duty unit is notified for deployment or redeployment. j. R-day. Redeployment day. The day on which redeployment of major combat, combat support, and combat service support forces begins in an operation. k. S-day. The day the President authorizes Selective Reserve callup (not more than 200,000). l. T-day. The effective day coincident with Presidential declaration of national emergency and authorization of partial mobilization (not more than 1,000,000 personnel exclusive of the 200,000 callup). m. W-day. Declared by the National Command Authorities, W-day is associated with an adversary decision to prepare for war (unambiguous strategic warning). time-sensitive targets|Those targets requiring immediate response because they pose (or will soon pose) a danger to friendly forces or are highly lucrative, fleeting targets of opportunity. Also called TSTs.
Military dictionary. 2000.