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If you want to check the watch's error over a week any of the websites should be good enough. I would expect that the latency would be pretty constant, and so will cancel out in any week to week comparison. And if it's off by a second- that's negligible compared to the drift in most watches over a weeks time.
I use the shortwave radio broadcasts from the National Bureau of Standards because I happen to have a bunch of radios, but that's not practical for most people. My second choice would be the desktop digital clock I got for my wife, it synchronizes to NBS radio transmissions.
Another possibility may be the time on the cable modem. I would assume that the modem is synched to a good clock in order to handle the digital data stream.
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