What does the "D" signify in D-Day?

So if D+3 means 3 days after D-day, won't D+5 (D5) mean 5 days AFTER D-Day?

https://history.army.mil/faq/ddaydef.htm

>>6689258

1-Year Delta = June 11, 2019

>>6689278

U.S. Coast Guard tweet included EAGLE and a picture of EVERGREEN.

https://twitter.com/USCG/status/1136633937214787584

>>6689504