On Disney's site Home>Park Hours they list Saturday's hours 12:00am-11:59pm (Fridays 6am-11:59pm), so if you had a one day pass, arrived 12am then I would think you could insist on staying on through the entire rest of the day for 23:59 hours...it's the same day, it's one day, it's in THEIR schedule...can't follow the "rules" any better than that.
I was REAL upset when my co-worker, who had asked for my Disney advice for their 1st trip ever to WDW, was treated like crap at customer service, and walked into the park upset (and $20 poorer) over it. They were doing 3 days, bought a 4-day FL pass, and had expected to do Epcot two days, then Animal Kingdom the 3rd, then use the 4th day to go to the Magic Kingdom that evening. I told him that they might give him trouble since the 4-day pass was 1 ticket and didn't allow hopping. With this in mind he purposely asked the Disney ticket seller on the phone if he could do this and was told YES (adding the 4th day was $20 cheaper than turning the 3 day into a hopper). However when he went to MK he was denied and management argued he should have paid extra for the hopper, which he was forced to do (now he was $20 poorer and did not havie the opportunity to hop on the previous days even though he was forced to pay for it). I consider what he did as being a smart customer, and saw nothing wrong or improper with it. The only problem was Disney's inability to handle that circumstance, and the manager should have made it work instead of stealing $20 from him, essentially robbing him of 2 hopper days and leaving a 1st time guest upset.
