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Old 11-04-2010, 03:17 PM
Ron Johnson
 
Default An experiment about file timestamp

On 11/04/2010 10:23 AM, Rodolfo Medina wrote:

Chris Jackson<c.jackson@shadowcat.co.uk> writes:


File timestamps are (or at least should be) stored in UTC. It's the
display of them that's affected.




But I did the following experiment: on a computer with system time set to UTC,
I created a file at 14:43 UTC. Then I copied it via rsync and ethernet cross
cable to another PC with system time set to GMT, one hour late respect to UTC.
I expected that, on the 2nd PC, the timestamp was displayed in the local time,
i.e. 15:43; instead, it appears as 14:43 as well. (For the copy I used the -t
option.)


Eh? Is not UTC, for all practical purposes, the same as GMT?

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Old 11-04-2010, 04:38 PM
Rodolfo Medina
 
Default An experiment about file timestamp

On Thu, 04 Nov 2010 15:23:13 +0000, Rodolfo Medina wrote:

>> [...] I did the following experiment: on a computer with system time set
>> to UTC, I created a file at 14:43 UTC. Then I copied it via rsync and
>> ethernet cross cable to another PC with system time set to GMT, one hour
>> late respect to UTC. I expected that, on the 2nd PC, the timestamp was
>> displayed in the local time, i.e. 15:43; instead, it appears as 14:43 as
>> well. (For the copy I used the -t option.)
>>
>> So, according with this experiment it is not true that the displayed
>> time is in local format.
>
> I think this may cause serious errors: in fact, when someone read the
> timestamp on the 2nd PC, he would believe that the file were created at 14:43
> of the GMT time, which is wrong: in fact, it was created at 15:43 GMT = 14:43
> UTC.


Camaleón <noelamac@gmail.com> writes:

> (...)
>
> Mmm, nope :-)
>
> I think you didn't get the whole picture.
>
> Look, it is very well explained in this Gentoo FAQ:
>
> ***
> Consistent times on FAT filesystems over the whole year
> https://forums.gentoo.org/viewtopic-t-579915-start-0.html
> ***


But the above `experiment' has nothing to do with vfat partitions: both systems
are linux ext3. What I don't understand is how can a user from the 2nd system
know that the file has been created at 14:43 UTC and not at 14:43 of its local
time, since all files of his filesystem are displayed in the local time.

Rodolfo


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Old 11-04-2010, 04:44 PM
Chris Davies
 
Default An experiment about file timestamp

Rodolfo Medina <rodolfo.medina@gmail.com> wrote:
> But I did the following experiment: on a computer with system time
> set to UTC, I created a file at 14:43 UTC. Then I copied it via rsync
> and ethernet cross cable to another PC with system time set to GMT,
> one hour late respect to UTC.

GMT is not (and never is) one hour later than UTC. For most purposes
you can consider GMT = UTC.

Corollary: if you've managed to get one system on UTC showing a time
one hour distinct from another system running on GMT then you've got a
clock wrong somewhere by one hour.

Chris


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Old 11-04-2010, 05:01 PM
Chris Jackson
 
Default An experiment about file timestamp

Rodolfo Medina wrote:

> Chris Jackson <c.jackson@shadowcat.co.uk> writes:
>
>> File timestamps are (or at least should be) stored in UTC. It's the
>> display of them that's affected.
>
>
>
> But I did the following experiment: on a computer with system time set to UTC,
> I created a file at 14:43 UTC. Then I copied it via rsync and ethernet cross
> cable to another PC with system time set to GMT, one hour late respect to UTC.
> I expected that, on the 2nd PC, the timestamp was displayed in the local time,
> i.e. 15:43; instead, it appears as 14:43 as well. (For the copy I used the -t
> option.)
>
> So, according with this experiment it is not true that the displayed time is in
> local format.
>
> I think this may cause serious errors: in fact, when someone read the timestamp
> on the 2nd PC, he would believe that the file were created at 14:43 of the GMT
> time, which is wrong: in fact, it was created at 15:43 GMT = 14:43 UTC.
>
> What do you all think?
>
> Rodolfo
>
>


GMT, as far as a computer is concerned, is the same as UTC. The
difference is that GMT is a solar time and may be up to a second
different, however since computers don't make solar observations,
they're the same in implementation, and many people use them loosely to
mean the same thing.

http://en.wikipedia.org/wiki/Coordinated_Universal_Time

Paragraph 4 of the introduction, plus discussion under "History".

Are you thinking of British Summer Time, a form of daylight saving time,
which ended last weekend anyway so the UK is now on GMT?

The question about FAT filesystems is a different one as Camaleon
observes, however. I misunderstood in my original mail.

--
Chris Jackson
Shadowcat Systems Ltd.


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Old 11-04-2010, 05:16 PM
Camaleón
 
Default An experiment about file timestamp

On Thu, 04 Nov 2010 17:38:43 +0000, Rodolfo Medina wrote:

>> I think this may cause serious errors: in fact, when someone read the
>> timestamp on the 2nd PC, he would believe that the file were created at
>> 14:43 of the GMT time, which is wrong: in fact, it was created at 15:43
>> GMT = 14:43 UTC.
>
>
> Camaleón <noelamac@gmail.com> writes:
>
>> (...)
>>
>> Mmm, nope :-)
>>
>> I think you didn't get the whole picture.
>>
>> Look, it is very well explained in this Gentoo FAQ:
>>
>> ***
>> Consistent times on FAT filesystems over the whole year
>> https://forums.gentoo.org/viewtopic-t-579915-start-0.html ***
>
>
> But the above `experiment' has nothing to do with vfat partitions: both
> systems are linux ext3.

Ext3 does not expose that "time-moving" behaviour, just FAT.

> What I don't understand is how can a user from
> the 2nd system know that the file has been created at 14:43 UTC and not
> at 14:43 of its local time, since all files of his filesystem are
> displayed in the local time.

To avoid confusion, run "date" command on both computers, then create a
new file in computer 1 ("touch test_file") and then copy/paste the file
into computer 2. After that, put here the output of "stat
test_file" (runned from both systems) so we can get the full story >:-)

Greetings,

--
Camaleón


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Old 11-04-2010, 07:00 PM
Rodolfo Medina
 
Default An experiment about file timestamp

Chris Davies <chris-usenet@roaima.co.uk> writes:

> Rodolfo Medina <rodolfo.medina@gmail.com> wrote:
>> But I did the following experiment: on a computer with system time
>> set to UTC, I created a file at 14:43 UTC. Then I copied it via rsync
>> and ethernet cross cable to another PC with system time set to GMT,
>> one hour late respect to UTC.
>
> GMT is not (and never is) one hour later than UTC. For most purposes
> you can consider GMT = UTC.
>
> Corollary: if you've managed to get one system on UTC showing a time
> one hour distinct from another system running on GMT then you've got a
> clock wrong somewhere by one hour.


Yes, that was it. So the experiment only proves my miserable error...

Sorry, thanks to all

Rodolfo


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Old 12-02-2010, 08:41 AM
Lisi
 
Default An experiment about file timestamp

On Thursday 04 November 2010 16:17:29 Ron Johnson wrote:
> On 11/04/2010 10:23 AM, Rodolfo Medina wrote:
> > Chris Jackson<c.jackson@shadowcat.co.uk> writes:
> >> File timestamps are (or at least should be) stored in UTC. It's the
> >> display of them that's affected.
> >
> > But I did the following experiment: on a computer with system time set to
> > UTC, I created a file at 14:43 UTC. Then I copied it via rsync and
> > ethernet cross cable to another PC with system time set to GMT, one hour
> > late respect to UTC. I expected that, on the 2nd PC, the timestamp was
> > displayed in the local time, i.e. 15:43; instead, it appears as 14:43 as
> > well. (For the copy I used the -t option.)
>
> Eh? Is not UTC, for all practical purposes, the same as GMT?

Yes!!

Lisi


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