Beijing 144 hour Visa Free

Hi,
My family plan to travel from Melbourne to Beijing on Tuesday the 16th of April. Our flight would arrive into Hong Kong at 5.20pm on the 16th of April. We would then be in transit for 3 1/2 hours in Hong Kong airport. Our connecting flight to Beijing would then depart Hong Kong at 11pm on the 16th of April and arrive into Beijing at 2.15am on Wednesday the 17th of April. We would like to stay the maximum amount of time in the Beijing area. I know that the 144-hours starts at midnight the day after we arrive in China. I would like to know if our 144 hour free time would start Tuesday midnight or Wednesday midnight? It depends though if China counts the Hong Kong arrival time (5.20pm on the 16th of April) as the beginning of the 144 hour transit visa or if it counts the Beijing arrival time (2.15am 17th of April)as the beginning of the 144 hour free transit. I plan to book a flight for my family from Beijing direct to Singapore on the last day of the 144 hour free transit.
Thanks for your help, it is much appreciated.