this is from the printable postage FAQ:
What is the significance of the postmark date on the Wrapper Settings page? Does it matter what date I choose there?
- Yes, it is important to choose the date properly--you need to mail the book by that date.
- You cannot change the postmark date after you have downloaded/printed the wrapper.
- If you mail AFTER that date, you can go to the PO and have a new metered $0.00 strip with that day's date applied to the package.
- It is okay to mail it earlier than the date; your package may be returned to you if you mail it after that date.
- If you put it into a collection box after the last day's scheduled collection, then the package will be accepted with the previous day's date when the next collection happens on the next day.
- If you are into legalese, here are the USPS regulations on this ("indicia" = postmark):
Mailpieces bearing a complete date in the indicia must be deposited or presented on that date, except that pieces entered after the day's last scheduled collection from the post office or collection box may bear the actual date of entry or the date of the next scheduled collection from the post office or collection box. When authorized by USPS, presort mail accepted after midnight may bear the previous day's date. If the mailer knows that the mail is not to be deposited or presented on the date in the indicia, the mailer must use a date correction indicium under 4.4.1. (from DMM, 4.5.2 Mailing Date Accuracy and Mailing Periods)