On a properly-working unix system, the hardware clock is
set to UTC. In theory, every unix system in the world has a
hardware clock that reads the same value at the same time. The
localtime file is a set of rules that adjusts your UTC clock
value to whatever local wall clock time should be. In a lot of
Europe, it will be UTC+1 plus whatever the rules are for where
you live which is why the city names exist. I live about 800
miles or over a thousand kilometers from Chicago, but that is
the city those of us in the US-Central time zone set up as the
localtime file because Chicago keeps exactly the same time as
everybody else in this time zone.
In the UK, you could just set things up for UTC, but you
need the rules file Europe/London to automatically set your
clock forward an hour on the last Sunday in March which is March
30 this year.
In some parts of the world such as the Northern
Territory of Australia and at least parts of India, the time
correction is designed to be closer to Solar time so while the
hours all change at the same time for most of us, their hours
change on our half-hour.
We had a student working for us a few years ago who was
from India and he told me that his home was 9-and-1-half hours
ahead of Central time. I don't have any idea if this value is
constant all year but it most likely varies when each country
adjusts it's clocks for daylight shifting, whatever you like to
call it.
Still, if you dug in to the computer of a resident of
India or the Northern Territory of Australia, red their hardware
clock and then immediately read the hardware clock of a resident
of London or Las Angeles, they would, in theory, read exactly
the same count.
Of course, if you have a computer that makes use of two
operating systems such as Windows and Linux, you may have to
forego all that great automation and set your hardware clock to
local time and remember to reset it when the clocks change.
Actually, I think Windows now also uses the UTC plus
local rules method of keeping it's time.
Anyway, I have some old Linux systems which are all
using America/Chicago except for 1 which is using posix/London
which will hopefully make cron run as if I were, in fact, in the
UK.
To answer Ron's question, the time stamps on files are
based on the hardware clock to the best of my knowledge.
When you look at one, a binary value reflecting what the
hardware clock was is neatly converted to the text you see so,
if the rules change, your older files might appear to have been
made an hour sooner or later than they really were made unless
the rules file remembers when the rules changed and adjusts for
that.
I bet you never thought it was this complicated.
I could be wrong about it all, but I think most of this
is accurate.
Martin
Ron Leach writes:
> Hadn't realised any of this, so thank you. If 'system time' and 'desktop
> time' differ - such as is suggested - what 'timestamp' is put on files
> when
> they are created? And does this differ whether the files are on NFS, and
> on
> another server? Is there an implication, here, that if a site uses desktop
> and system times (that differ) on one machine (a laptop, say), then 'all'
> the machines on the network, especially the file servers, must be
> configured that way?
>
>
>
> (I could see this being an issue for timestamps on backups across NFS, and
> on Dovecot which is very sensitive to time changes.)
>
>
>
> I've not yet read the tzdata readme, which may discuss some of this, but I
> will do so, likely after office hours, though.
>
>
> regards, Ron
>
>
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