Re: system time - thank you

[daniele pendenza <danielependenza@yahoo.fr>]

Hi !

and Thank you Bob for the kind answer. It was my mistake not to read  
the documentation in depth. So your suggestion was a great help. Now I 
understand the results of hwclock,
Just to make it clear to everyone interested - also in the future :

1 - 'hwclock --show' :  reads  by its own the content of '/etc/adjtime' 
(if this file does not exist the dafault(*) is localtime) to understand 
if the RTC is in UTC or LOCALTIME than it shows, after a conversion if 
necessary, the time in localtime.

2 - ' hwclock --show --utc' : _I_ explicitly_ say - no matter if this is 
the truth or not - to hwclock  that the RTC is in UTC, than it shows the 
time got from the hardware, after a conversion.

3 - 'hwclock --show --localtime' : _I explicitly_ say - no matter if 
this is the truth or not - to hwclock  that the RTC is in LOCALTIME, 
than it shows the time got from the hardware,without a conversion.

By the way a question arises:  why the default in (*) is localtime ... 
It seems that when I first wrote the post the system '/etc/default/rcS' 
has been configured with 'UTC=yes' ... mmm ... odd, isnt it ? ;)

cheers and Thank u again!

daniele



Bob Proulx wrote:
> daniele pendenza wrote:
>   
> > So I think this is the general picture :
> > 1 - when the system starts  the value of the system time is defined 
> > taking into account the value of the RTC time the value of the UTC 
> > constant retrived at startup and the timezone information.
> > 2 - when the system is up and running the system time is "the time that 
> > matters" - as you said - and, when requested, it is presented to users 
> > after the "timezone shift" eventually.
> > 3 - when the system is shutdown the system time is _saved_ after 
> > calculations based on the same informations as in line 1.
> >     
>
> All correct.
>
>   
> > If all that I wrote is correct : why the command ' hwclock --show 
> > --localtime' shows me the UTC time and 'hwclock --show --utc' shows me 
> > the localtime ?
> >     
>
> The --utc and --localtime options are not to affect the display but to
> configure how hwclock reads time from the hardware clock.  Those
> options tell hwclock whether the hardware clock is kept in UTC or in
> localtime.  The --show always displays in localtime.
>
>        --show Read the Hardware Clock and print the time on Standard
>               Output.  The time shown is always in local time, even if
>               you keep your Hardware Clock in Coordinated Universal
>               Time.  See the --utc option.
>
>   
> > I set up UTC=yes and the timezone is Europe/Rome and I don't have 
> > windows... uh
> >     
>
> Then hwclock --utc would be the correct option for you since the
> hardware clock is set to UTC.  If you use --localtime then hwclock
> will wrongy think that the hardware clock is in local time and apply
> an undesired correction factor.
>
> Generally userland applications should use date instead of hwclock to
> get time information.  The hardware clock interface will be slower and
> more resource intensive because this is through the kernel and from
> the hardware.  Plus IIRC while doing this hardware clocks may skip
> clock tick interrupts.  Plus the hardware clock is not corrected for
> drift.  The system clock should be running an NTP daemon and would
> always have a corrected value.  Because of these things it is almost
> always better to use the system clock over the hardware clock.
>
> Bob
>