Laundry Dilemma

To book one of the two laundries where I'm staying you use this lock that your room key operates and latch it onto to time slot. It is a grid of slots, with the days of the month across the top (1-31) and the times down the side, 6-9, 9-12 etc.
my dilemma:
I just went down to book the laundry, and what do you know it, tomorrow morning 1st,9-12 is free. Sweet!! Since it's my day off after working the weekend. But then I look across and someone has used the slot 31, 9-12. So has someone not realised that it is SEPTEMBER with only 30 days and thought tomorrow is the 31st?
Is it a case of tough! Use your brain next time? Or should I have left the slot blank, knowing that someone has probably made a mistake? and there are others that booked the 31st also!
Tomorrow there may be a war in the laundry, comments please...