stop over at Guangzhou Baiyun Airport

I'm taking a flight from London heathrow to hong kong, stopping over at Guangzhou Baiyun Airport.

I'm not completely clear on the '72 hour will start from the 12:00am on the next day when you transit via Guangzhou Airports' rule.

If I arrive Guangzhou Baiyun Airport at 18:30 on a tuesday and catching my connection flight to hong kong at 15:55 on that Friday. Will I still be able to get my 72 hours-visa free transit?

Mant thanks!
Ella