Re: crontab failure for daylight savings

[Camillo Särs <ged@iki.fi>]

Olaf Dietsche wrote:
> If you go to daylight savings, the clock jumps from 2am to 3am.

No it doesn't.  The clock ticks along as it always has done, in UTC.  The 
timezone simply changes, which should be no big deal for timezone-aware 
applications.

> The reverse happens, when you go back from daylight savings to regular
> time: the clock jumps from 3am to 2am. This means, all jobs between
> 2am and 3am run twice. I don't know, wether some cron implementation
> compensates for daylight savings.

From woody cron(8):
       Special considerations exist when the clock is changed  by
       less than 3 hours, for example at the beginning and end of
       daylight savings time. If the  time  has  moved  forwards,
       those  jobs  which  would  have  run  in the time that was
       skipped will be run soon after the change.  Conversely, if
       the  time  has moved backwards by less than 3 hours, those
       jobs that fall into the repeated time will not be  re-run.

The root cause of this is of course that the crontab format is historic, 
and does not indicate what timezone should be used when interpreting it. 
Such design is always error prone.  Good design would include a well 
documented way of including the timezone.  Well, the above quote might even 
qualify. ;-)

Of course, using a 12-hour clock is also very error prone, but fortunately 
cron at least uses a standard 24-hour clock.  No am/pm mess at least.

Camillo