Found a decent note on how to setup NTP on Plan9 on the bottom of that page:
Last, but not the least, you may also want to edit/change the TIMESYNCARGS in your /rc/bin/termrc or /rc/bin/cpurc; for example, to use an NTP service, set it to:TIMESYNCARGS=(-n pool.ntp.org)
There is a command aux/timesync that is start from /rc/bin/termrc It takes a DNS name of a time server as an argument. If you modify the value set as default and reboot you should be good.
I ran it from the command line as a test and it worked
term% aux/timesync -n pool.ntp.org