The success of the invasion of Normandy in World War II depended on having the right weather (mild) and the right tide (to carry the ships up onto the beach). When military strategists were planning the invasion, they didn’t know when that would occur, but they had to be able to plan for what would happen before and after the troops landed. So they called whatever day the landing was going to occur on, D-Day (D cleverly meaning "day"). Then they could talk about D-3 (three days before the landing) or D+2 (two days after the landing) and plan out the entire invasion around an unknown date. Similarly, the exact time was H-Hour (although this depended on which assault force was involved). For example, on D-1, bombers would bombard northern France; on D + l, block ships were to be sunk off the beaches to form breakwaters; and at H+6, more troops were brought onto the beach for re-inforcement. Once the actual date and time of the invasion was announced, everyone knew exactly what they had to do and when, and the invasion could run according to plan (to the extent that war ever does).
This use of a letter to stand for a specific but unknown time is the same as in algebra, where x and y stand for specific but unknown numbers. In this case, D+3 = June 9, so D must be June 6. Similarly, if x+3=9, then x must equal 6.