These solutions involving measuring the crystal still have a "belling the cat" (good idea, say the mice, now which one of us will do it ?) about them, you will need a VERY good crystal timebase to sync things up,probably ovened crystals etc etc. 1 sec /month is 0.5 ppm ! Over temperature....

Charles' dial up service idea sounds good for US users, maybe a simple system based on NMTP is possible too ?

Steve