Not a great fan of that router. After six months I was fortunate enough in that it packed it in and BT replaced it with their Voyager 2110 which I find much better.The only problem I found when switching off the router is I have to set the time setting on the router again after I switch it on.
I know there is an option in the router settings for a "time server" but I don;t know how to do this. I have read the manual - ZyXEL P600.