[lug] laptop time

[John Karns]

On Fri, 23 Feb 2001, Timothy C. Klein said:

>Ok, I am confused now by the terms.  Aren't the RTC and the CMOS clock
>the same thing?   Or is the RTC the one in the kernel? 

It can be a little confusing.  A less ambiguous way to refer to them are
system clock vs hardware clock.  I believe that the RTC is synonymous with
the hardware clock and CMSO clock.


>Either way, if you set the clock on your system board (BIOS, CMOS, RTC,
>whatever we call it) to UTC (or GMT as it used to be called), Linux will
>be happier about your machine's time.  By UTC I means Universal
>Coordinated time, or the time in Greanwich (sp?), England (currently
>-0700 from MST).  Your time will be goofed up, though, if you don't run
>tzconfig and let the kernel know what offset to apply to UTC to get your
>machine to display local time (Mountain Standard Time for me).
>Thus, on my machine, I get:


>#rdate
>Fri Feb 23 00:18:50 2001 MST  # This is my time in Denver
>
>#rdate --utc
>Fri Feb 23 07:18:59 2001 UTC  # This is the time to the linux kernel and BIOS

What about the timestamps on the filesystem.  What I would't like is
having to deal with GMT in the file timestamps.  I presume that cron would
be referenced to the local time and not GMT - correct?

Then you specify GMT for the RTC in the kernel config as well?

RTC stores time in GMT
CONFIG_APM_RTC_IS_GMT
  Say Y here if your RTC (Real Time Clock a.k.a. hardware clock)
  stores the time in GMT (Greenwich Mean Time). Say N if your RTC
  stores localtime.

  It is in fact recommended to store GMT in your RTC, because then you
  don't have to worry about daylight savings time changes. The only
  reason not to use GMT in your RTC is if you also run a broken OS
  that doesn't understand GMT.


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John Karns                                              jkarns at csd.net