I like the Navy Clock:
http://tycho.usno.navy.mil/simpletime.html

No Flash.

That's a good one, too, but I like the sunlight map on Time.com. Either way, you'll get the right time.

That's a good one, too, but I like the sunlight map on Time.com. Either way, you'll get the right time.

What is the +/- tolerance for B1 per month (or year)?

It's COSC certified, so it's -4/+6 secs per day.

I think this -4/+6 applies to mechanical watches only, for quartz should be much lower.

I think this -4/+6 applies to mechanical watches only, for quartz should be much lower.

What is the +/- tolerance for B1 per month (or year)?

Plus, time.gov has the feature of letting you how how accurate the reading is based on your connection speed. I get mine to "within 0.2 sec." and have had it as bad as 3 sec or so off when the site or my connection are slow. With the other sites you have no idea how well synced you are.

Another good point. I've never seen time.com worse than 0.3 seconds off, but if it were 3 seconds, that would be pretty significant. I would want to know that before setting my clocks by it, or checking the accuracy of my atomic clock. Interestingly, I compared my atomic clock, my Sprint cell phone, and my TAG Heuer quartz side by side with time.com when it was within 0.2 seconds, and all three changed within blinking time of each other. (I had set the TAG by the atomic clock maybe a week earlier.) Conversely, my computer's clock is off by about three seconds.

Let me further add that you can bookmark the Navy Clock on your fancy phone, and it'll work. Time.gov doesn't work on smartphones, 'cause it's Flash-based, and Flash doesn't work on phones for no good reason.