What can I do if my departure is scheduled at hour 73 of my visit

Assuming you’re in transit, it’s not a problem because it’s 144 hours transit without visa in Beijing now.

Rob is correct for Beijing transit but one thing to keep in mind for other ports is that the 72 hours doesnt start until midnight so you would not be required to leave until 11.59PM on Sunday. Additionally, you would pass through Immigration long before the time you nominate (ie 3 hours or thereabouts before departure)...Enjoy!

The 72 hours starts at midnight the following day in most cities, but not in all. In Guilin (KWL), Kunming (KMG), Harbin (HRB) the time is strictly 72 hours. Moreover, the time is calculated from scheduled arrival time to scheduled departure time, and not on the time that you pass immigration.