We fly from London to Chongqing (arrive at 16.40 27th April)

and have a connecting flight to Beijing (arrive at 22.40 27th April). Will our 72hours start counting from our arrival times are either of these airports or will it start at 00.00 on the 28th of April?

We then have a flight from Beijing back to Chongqing (depart Beijing at 16.00 30th April) then Chongqing to New Zealand (depart Chongqing at 00.15 1st May). Will our 72 hours end when we depart Beijing, since we only transit to Chongqing, or when we depart Chongqing?

Thanks.