time change - a warning

[John Abreau]

Seth Gordon <seth at genome.wi.mit.edu> writes:

> Warning: when Daylight Savings Time ends, if you set your computer's
> clock back using Linux, the time on Linux may start acting strangely.

Ick! Why would anyone want to much with daylight savings time manually?
The basis of the Unix clock is merely the number of seconds since the 
epoch, and the concept of Daylight Savings Time is supposed to be a higher
level concept implemented in the functions that print the time in various
formats. If you muck with the system clock to track daylight savings, 
you're just asking for trouble.

Linux behaves in the proper Unix manner when you set the hardware clock 
to UTC and define your local timezone. In effect, with Daylight Savings 
you actually have two completely separate timezones, and date(1) and
strftime(3) do the Right Thing based on which is active for a particular
clock value.

The two time zones (here on the East Coast) are EST (Eastern Standard 
Time)
and EDT (Eastern Daylight Time). On any particular day, for instance, 
7:00 pm EST == 8:00 pm EDT. The only real difference is, we switch which 
one we're using twice a year, so for any given second, one of them is in 
use
and the other only exists in a theoretical sense.

So the date(1) command shows for the following clock values:

    1018162799 => Sun Apr  7 01:59:59 EST 2002
    1018162800 => Sun Apr  7 03:00:00 EDT 2002
    :
    :
    1035698399 => Sun Oct 27 01:59:59 EDT 2002
    1035698400 => Sun Oct 27 01:00:00 EST 2002
    :
    1035702000 => Sun Oct 27 02:00:00 EST 2002

The commands I used to determine this were of the forms

    date -d '2002-10-27 06:00:00 UTC'
and
    date -d '2002-10-27 06:00:00 UTC +%s'


-- 
John Abreau / Executive Director, Boston Linux & Unix 
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   Some people say, "The enemy of my enemy is my friend."
   I often respond, "When elephants fight, it's the grass
   that gets trampled."



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