I don't know what the included save is like. But the game won't save on it's own, you have to pick one of the two save...
I always choose the bottom left and I'd say that somehow the saves were mixed up for my stats were all which I thought them to be. The same applies to the relationship status. Meanwhile after I (accidentally) deflowered her (urethra), her insane stats were revealed.
I redownloaded the game and will tried to replicate it... and I can say: The included save starts at Day 120 before the confession but with the high girl stats (after disappointing her since I could only quit the H). Quitting and starting a new game, playing a day without doing anything, save, quit and continue resulted in at least bouncing me to day 20. Deleting the Save folder and clicking Continue started a new game so I quit. Starting a new game and doing the same till I could save and quit... I found myself on Day 20 (without visible progress) again. If you are confused by that then at least we are two.
I hope if I play normally again beyond the 20th day, I will not find myself utterly devastated and demoralized by the corruption. I hope that these big jumps are limited to the save(s, there are two, I hazard to guess that one is supposed to be persistent, serving as pseudo-NG+ for the sister's stats) and the 20 Day Skip is a hidden "I don't wanna wait until work is released"-skip.
