@Dominions SonIt could be "Day Zero" because it is not a Full Day.
Thus "Day-1" is the First FULL Day.
For example, the Day of an Invasion is "D-Day", and subsequent days are D+1, D+5, D+10, etc.
The most famous D-Day was June 6th, 1944, Operation Overlord, the invasion of Normandy France to become the liberation of (Wester) Europe {since the Allies had landed in southern Europe in 1943}. the first full day of combat was June 7th D+1. Plans for landing reinforcements or building the Mulberry temporary harbors were scheduled for specific days after the initial landing. This was important because the landings were scheduled for June 5th but got pushed back by 1 day due to weather.
Thus, the Allied plans did not have to be changed because they were based on when the Allies actually landed, not the day they were scheduled to land.
The Cobra Breakout began on July 28th D+52
To reiterate, an incomplete day could be termed "Day Zero" and Day 1 would be the first complete day after.