D for Departure date?
just a thought.
Cheers
Peter

The "D" in D-day stands for "Day". Wierd right? Because it would read Day-day. Well, it's so. It's the actual "Day" in which a planed action takes place.

Further more, D-DAY, is the date for a planned action. The term enables planners to establish sequential priorities for activities before and after an operation's starting date, which may be unknown, may be designated but secret, or may be subject to change. For example, when an invasion is contemplated, the date for beginning the action is called D-day; planners can then schedule an air bombardment for D minus 1 (D - 1), the day before the target date, and the commitment of additional troops on D plus 1 (D + 1), the day after. Preparations for a complicated operation are keyed to an established schedule requiring certain acts by D - 90, others by D - 60. In the same way, follow-up activities and desired or expected events may be scheduled for D + 10, D + 30, and so forth.

Similarly, H-hour designates a specific time for a planned action, such as the landing of the first troops. M-day usually means mobilization day, the date for certain planned actions to take place in a specified sequence in the event of possible enemy activity.