Why is the NTP controlled computer clock two minutes ahead

clockdate timentpsync

The clock in my computer is configured to be synchronized using NTP. To verify this I have tried two NTP clients using various NTP servers. My computer and the NTP clients are in complete agreement about the current time even across a wide range of NTP servers.

I also have a GPS and my national phone company provides an accurate clock available by calling a specific phone number. Both my GPS and the phone company agrees on the current time. However, my computer is almost precisely two minutes (or 1 minute and 59 seconds) ahead of what I believe to be the "real" current time where I live.

Why is my computer two minutes ahead? I realize that synchronizing clocks using the internet may not be entirely accurate as there is latency, but two minutes is a very long time on the internet. Is NTP really two minutes ahead? I'm running Windows 7 and live in the time zone UTC+1, but I don't think that is important in understanding my problem.

Best Answer

GPS time isn't the same as UTC, it's just very close.

GPS is a very accurate source as far as time differences go, but as it doesn't take into account leap seconds it hasn't actually been in sync with UTC since January 1980. However it's only about 15 seconds ahead, so that doesn't account for your 2 minute difference.

If your other time sources are based on the international atomic clock standard (TAI, Temps Atomique International), then they're also not set to UTC, not because they're inaccurate, but more because UTC is inaccurate in comparison. They don't take into account leap seconds, and their time is based on ticks of the SI standard 'second' rather than being based on the rotation/orbit of the Earth as UTC is. They're about 34 seconds ahead of UTC. Still doesn't account for your difference.

See here http://leapsecond.com/java/gpsclock.htm or here http://en.wikipedia.org/wiki/Global_Positioning_System#Timekeeping for more info.

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