10 day F visa - how long can I stay in China?

Hi all. I have a business trip to Beijing which requires a 12-day stay (I arrive on Sep 18th 10 AM and depart on Sep 29th 12 AM). Tickets are already booked and non-refundable. But the embassy gave me a 10 DAY F!
If the duration of stay is counted from the next day after entry, my 10th day will be Sep 28th, not 29th. Should I ask for a prolongation or my present F is fine? Can anybody help me here? Thank you!