If timed correctly it can win the game on day one, I timed a 3 hour turn so that I had 1 one hour turn left at the end of the day and in that longt turn I put one of the objectives under my FoF from 2 units as I extended my line, next turn they had one attack chance but couldn't dislodge both unoits and with night being the next turn I "won".
The Confederates can't win on 1 July:
"At the end of any Union action phase on 2 or 3 July, if the Union
player does not fully control all 3 objectives, and he does not declare
Attack general orders for his next turn, then the game ends and the
Taking an objective right before nightfall can indeed win on 2 or 3 July, but not on 1 July. If an objective were taken right before nightfall on 1 July, the Union would have the morning turns on 2 July to try to retake the objective.