If sync to say 20mSec on average is OK - I did so using a network with unicast and broadcast datagrams. A gateway node broadcasts its tick count (time) every few seconds to all in-range nodes. Freeware Protocols are:
http://www.airspayce.com/mikem/arduino/RadioHead/
for low cost sub-GHz transceivers, e.g.,
http://www.anarduino.com/details.jsp?pid=145
SFE doesn’t sell such boards.