score:2
The immigration officer at Shanghai airport told me that they calculate the 24 hr period from the next day after arrival. And after arrival on May 1 morning, I should just leave by May 3 (which strictly speaking, should have been May 2, but he said May 3).
Upvote:4
The information provided by the Guangzhou Airport authority indicates that (emphasis mine):
Most nationalities require a visa to enter China. A visa is not required, however, if you are in transit for a period not exceeding 24 hours as determined by scheduled flight arrival and departure times, and already have confirmed onward tickets. The FAQs reference the same metric for other transit-without-visas, 72- and 144-hour, i.e., scheduled flight arrival and departure.
That noted, @JonathanReez offered a key observation: you may have difficulty boarding your outbound flight, which you may have overcome before departure, as the airline observes the visa rules of the countries to which and through which you'll travel.
It is not likely that you could surmount the requirements for an emergency visa on arrival, as Chinese officials grant those to individuals whose destination is China. You would have to prove that a genuine emergency prevented you from applying in advance for a visa and have
Your link to Travel China Guide has similar information, and Trip Advisor China Traveler offers an easy summary of visa-free transits.