Ours are run through a postage meter. They get a date when they go out. I don’t remember whether they get anouther date when they go back through the mail.
Technically, your city/town is breaking the law. The mail is supposed to be mailed on the date metered. If the are using it for mail to be returned, that is illegal. Someone returning an absentee ballot is dropping it in an ordinary mailbox where it is going to go through the postmark machine.