Starting time of "72 hours" free transit

Hello...I will be traveling to Chengdu in December on NZ passport and can enter on 72 hours free transit. However can someone please tell me when does the actual time start as I have read that If i arrive at 8.00 today, the actual time will start 00.01 the following day. Is this true ? I would like to make it sure. I called up the Chinese Consulate in Auckland and they said refer to our website !!