Verified answer

First visit, but it must be manually adjusted by a cast member. The default is date of purchase, since most folks purchase and visit the same day.

Here's a scenario: Grandma like to buy things in advance. She buys an Annual Pass for her grandson's Christmas present in September. Grandson can't get away from college to visit the park until Spring Break. It really isn't fair to penalize him six months of Annual Pass usage, so Disney can manually adjust the Pass to reflect the date he visits versus the date six months prior when the Pass was purchased.