Daily time sync to internet time reverts to weekly after three days

[Ed Harris]

Hi Seven forum, I have connected 2 windows 7 professional laptops to the internet solely so that they can update the time ( I need them synchronised but they are drifting about 2 seconds a day). I have set the specialpollInterval to 86400 to do this daily. I had to restart to get this to work. It then worked for 3 days before resuming updates on a weekly basis. The special poll interval is still showing 86400. I restarted again and it ran for three days before again reverting to weekly (probably - as it has not been a full week).
Can anyone suggest what is going on and how to fix?
I suppose I could go through the event log for the day it reverted but dont know what to look for.
Ed