[CentOS] System Time

Mon Mar 9 01:32:14 UTC 2020
Chris Adams <linux at cmadams.net>

Once upon a time, Pete Biggs <pete at biggs.org.uk> said:
> There's also a massive problem with
> signal strength in the UK - the (singular) time transmitter is in the
> middle of the country in Cumbria and in the south it's virtually
> impossible getting a signal any further than about 2 feet from a window
> - not a hope of getting anything in an office building!

There are different systems around the world (WWVB in the US for
example), and I don't think there's a system at all in many countries.
Also, putting a receiver inside a computer case would pretty much never
work for the low radio frequencies used by these systems, so there'd
have to be an external antenna (a lot of effort to go to when you could
just use network time sources).

Radio clock accuracy is typically in the 100ms range, so is good enough
for most people's computer clock usage.

> GPS times also have problems. They are very accurately wrong!  The
> atomic clocks on the satellites haven't been updated since they were
> launched, so no leap seconds.

That is not a problem - GPS time is defined as being continuous, unlike
UTC.  However, the GPS signal includes the UTC offset, which is updated
when UTC applies a leap second, so you can calculate correct UTC from
just the radio signal.  I'm not as familiar with the GPS alternatives
(Galileo, GLONASS, Beidou, and more), but I believe they'd all be the
same (a continuous time base, with offsets specified in the data).

Also, again, GPS signals are weak and require an external antenna.

I do have an external GPS receiver and external antenna hooked up to one
system at home, so I have a stratum-1 NTP server (probably accurate to
about 1µs).

Basically for most, the "chip inside the box to set the clock" is the
network chip. :)  If you need clock setting on a disconnected network,
you can get a dedicated time server.

Chris Adams <linux at cmadams.net>