Thanks for your response. I always used my WLAN-routers that way, i.e. do the PPTP/PPPOE on a PC with a "real" firewall and other nice things and use the router as a "dumb" machine. Time was up to now of no matter, but since the 825 can do time based activation of WLAN and I want to switch WLAN off at night, time should be accurate. Unfortunately the internal timer isn't very accurate, I was ~ 45' off after an uptime of 2 weeks.
Maybe I'll find a way to POST the HTTP request automatically to kind of sync the time or I'll have to dig into OpenWrt, maybe it can handle this.