The way I understand it is this:
The manifest will only print the day the labels are shown as shipped that day.
You have until I believe 9 pm to make a manifest for packages being shipped that day.
So, if you are printing labels the night before the day you are shipping, the label would show the actual shipping day, and you can print that manifest anytime on the day of shipping up until 9 pm
Not sure this is clear. I usually print my labels at night, showing the next day as the shipping date. I cannot print the manifest that night, but have to wait until the next morning. (But they want you to only do one manifest/day, so I print it as late as possible in the morning, before my carrier arrives at 9:30. So I can catch any morning orders on it.)