First of all, we must determine the location of Tonga. It is located in the southwest Pacific and west of the International Date Line. It is the first country in the world to start a new day. (Exact location~see picture) So Tonga is in East 12th District.
And Beijing is in the East 8th District~~Tonga is east of Beijing~~so the time is faster than Beijing, and the difference in one time zone is 1 hour~~so Tonga is 12-8=5 hours ahead of Beijing