I have been playing with http://time.is and it is suprising that I get the same result, within about 0.2 sec, on my laptop on ADSL and on my phone on 4G
Does this site use some sort of feedback from the client device to work out the latency?
Maybe a variant of Marzullo’s Algorithm ?
I don’t think it is all that sophisticated. 4G latency is low enough to give 200ms accuracy.
I could try disabling 4G and 3G, forcing GPRS only, and see what happens 
Except that my phone doesn’t support that… So I tried my T705 tablet and 3G on that gives me less than a 100ms (no visible) lag between that and the ADSL laptop.
Peter wrote:
I could try disabling 4G and 3G, forcing GPRS only, and see what happens
On that specific day and time, you could have streamed HD media until the provider reduces your high-speed data until the end of the month (do they do that in the UK?), which would have been 3 hours later. 
Must use the client. In About they say:
The precision depends on your internet connection and how busy your computer is.
and
Synchronization via Ajax ensures maximum accuracy.
It’s a little fun, but does it have any practical application? Windows clocks are always synced anyway these days.
The Windows NTP client is very rudimentary, and on a laptop (or any other machine that spends time turned off) the clock may not always be synced. I’ve noted laptops in particular are clock drift monsters when left asleep for more than a day or so.
but does it have any practical application?
I was using it to check a mechanical watch
I’ve noted laptops in particular are clock drift monsters
That’s because the version of the old Hitachi HD146818 which is implemented in PC chipsets has a crappy oscillator design, as well as draws way too much power. The market simply has no interest in anything better.
Synchronization via Ajax ensures maximum accuracy.
How does that work?
http://www.pool.ntp.org/en/
The best way I think is to find a primary server close to your place.
In France https://services.renater.fr/ntp/serveurs_francais
