Windows has a very good client for updating the System Clock via an Atomic Clock using Network Time Protocol. Unfortunately is has one big drawback, the default update period is seven days which for most people far too long.
This little utility will allow you to adjust the period, down to hours if necessary. I find that every six hours is adequate on my computer and that keeps the time to within one second.
You can also add your own favourite Timer Server here.
For UK readers I would recommend ntp1.npl.co.uk and ntp2.npl.co.uk. NPL being the National Physiological Laboratory at Teddington where the UK's Atomic Clock is located.
For other locations a quick search on Google will locate many other time servers, there are hundreds of them. Most universities have a publicly accessible Time Server.
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