when i did it i just walked in and didn't actually call attention to the fact that i was walking in before my appt. when the date difference was noticed i just kinda shrugged sheepishly and implored the guy politely to do it early and he said ok. i think it also depends on when you go. i've only ever done walk-ins (just to get it over with and probably because i had something to do at my actual appt time and day). the first, done years ago, was done early in the morning just when they opened and the second was almost at the end of the day but the center only had a few folks getting their biometrics done. i was in and out in 5-7 mins. both times, there was hardly anybody there so, i guess, that's why it was allowed.