Supposedly the FW is not directly connected to Internet (i.e., it is  only 
used to create a VPN connection) it seems there is no way to update the FW 
time.

In fact monowall specifies the reference of a  selectable UTP but in no way 
the system clock can be used for the same purpose, nor it can be 
done  manually.

Do I state it correctly or there is some feature missing in my picture?

Francesco