Most of my players have this in their config.ini:

[hijack]
;@MENUEXEC Set_Date_Time /rdate -s 10.0.0.2 &>/dev/null

This allows me to reset the date/time at will from the Hijack menu, using my internal server (10.0.0.2) as a time reference.

I think the same thing might also work from EXEC_ONCE, except then it would need a script that could be more clever about waiting for the network to come before issuing the command.

Cheers