Just run the RIOT gnrc_border_router example. I used the SLIP uplink but it shouldn't make a difference. (Ethos might use a different IP for the host theough)
make UPLINK=slip flash term
The client will connect to the (ASCII) /time endpoint of the CoAP server.
For this I used the FreeCoAP time_server example.
Make sure to compile with ip6=y dtls=n to enable IPv6 and disable DTLS.
The server is then started with
./time_server "fdea:dbee:f::1" 5683
fdea:dbee:f::1 was configured as an IP for the host computer by the SLIP startup script.
Just flash this demo to a node. It should periodically connect to the border router & host computer to fetch the time and print it, then go back to sleep.