Hi Xinke,
I noticed you haven't had a response on your question yet. Were able to get your WSN timing set up already? The nodes themselves don't actually keep a timestamp because they always need a gateway in the system anyway. The gateway is the part that holds the system time. The way to set the system time is through an NTP time server. This is mentioned on Page 14 of the 9792's user manual.
Regards,
Imtiaz Chowdhury
Project Manager
Green Running / Austin Consultants