When I tested Time.is, it told me the clock on my computer was 4.1 seconds behind. It also knew I was in Seattle, WA.
Further, you can find out what the exact time is right now at any of 7 million locations around the world, and compare them. The clock is truly global — you can adjust the first day of the week (Saturday, Sunday or Monday), the date format and the time format.
What clocks do you use? Any mobile apps? Share with us in the comments below.