I Don't Think Monrovia's Zip Code Is 91017

This is kinda bizarre. This Star-News article (https://goo.gl/7Le1Mv) says the "Monrovia Post Office will offer a special postmark to commemorate the rare event when the date, 9-10-17, will match the community’s ZIP code 91017."

Since when is Monrovia's Zip code 91017?

Update: Ah! USPS says "This ZIP Code™ used for a specific PO BOX", so I guess the special postmark is to honor PO Boxes.  https://goo.gl/v5i25d

- Brad Haugaard