It can, depends on how busy the parks/restaurants are. I had a Chef Mickey adr in November that I had changed the weekend before going, from 9:30 pm to 5:30 pm (so I thought!/ When I got there, at 5:25! i walked up to the desk and said we were there. She laughed and said "um, your adr was for 4:30!" Luckily, we were a small party and it wasn't super busy that week at WDW, so she got us in in about 30 minutes. Stressful though!