That is, the meter itself didn't "know" the current date.It was not at all uncommon for somebody to forget to advance the date and run some mail through on the previous day's date.We then had to run the envelopes through with zero postage and the correct date on the back of the envelope to show the PO what date we were really mailing them.This indicates that the imprint was made by a Pitney-Bowes meter, serial number 6829680.
Finally, in the box on the right-hand end of the [indicium] is the postage value (29 cents)." So the numbers at the bottom of the stamp make up an identifying code for the postage meter that stays the same regardless of the date, amount, or location of the imprint.
Your correspondent would have to have failed to advance his/her meter for several days to accomplish what you suspect was done, but it is certainly possible.
Backdating postage comments