This is correct but a bit more of the why:
Many times the planning of a military operation takes place without an actual start date. Some events are scoped based on the day that the operation kicks of. However, that day is not known. Think of it like project management.
Suppose there is a project kick off date ... some time in June. Before the project kicks off, I know that X days prior, something needs to be done. So D-DAY minus X is the start date for that effort.
Also consider that the planning for Normandy was started MONTHS before the actual landings.
I assume it’s the same logic NASA uses for its “T minus” countdown clock.