If you are using a regular MYW ticket at the parks, and you use that ticket as credit towards upgrading to an AP, the start date of the AP will be the first day you used your MYW ticket, NOT the date you did your AP upgrade. If you waited to upgrade until the last day of use of your MYW ticket, your AP would still be dated to the first date of use of the ticket, and that could be as far back as 13-14 days.
I don't know if OP is buying an AP with a ticket credit or not, but I wanted to mention the above because it could make a difference to some people who are considering an AP purchase to cover future WDW visits.