Generally, you shouldn't have any trouble as long as you're handing the package to USPS® within a few days of the date on the label... but it is at the discretion of the USPS employee, so be super nice about it 😉
UPS®, on the other hand, is much more lenient than USPS when it comes to your ship date. UPS prefers that you use your labels within 100 days, but if you hand your package to UPS after that timeframe, they'll simply charge you an additional $2 "Missing PLD Fee" and deliver the package like normal.
Unfortunately once you’ve purchased a label there is no way to edit it, so if you need to change the Ship Date you will have to request a refund for the label and start over.