As a general rule, all mail that has been franked must be posted on the day of franking. If the post is missed, the mail should be refranked and posted the following day.
Firms that use a lot of mail can save postage by buying a franking machine and printing a frank at a discounted rate rather than using normal postage stamps. It is a requirement, however, that the frank shows the actual day that the mail is collected. If the date on franked mail must be changed, simply put a black line through the original frank, and then print a new frank with a zero value.