[ILUG] Daylight savings time question

[Colm Buckley]

> This question has come up a lot recently. Can someone
> explain what the hell is going on! Does the BIOS have
> anything to do with it (do some do DST themselves).
> What does Windows do (if anything) to the hardware clock

Windows keeps the hardware clock at "local time" - ie : the same as you
see on your taskbar.  Linux tends to put the hardware clock at "real
time" - ie : it *doesn't* pop it an hour forward in the summer (although
there's a kernel config option to change this - see below).

It's important to remember that Linux (and Unix in general) has a more
sensible approach to time than 'doze does.  Under unix, the system clock
is simply "seconds since 00:00:00 Jan 1 1970 GMT", and increases
monotonically, with no jumps forward or back.  Everything else,
including time zones, summer time, leap seconds, whatever, is a function
of the locale; the system libraries know how to work out the local time
from the system time, given the locale information.  This way, there are
no sudden jumps in the system time to mess with dependencies and so on.

The problem arises when you switch back and forth between this nice
sensible Unix setup and Windows, which manipulates the system clock to
keep it the same as 'wall clock' time.  During the summer, in this
locale (GMT/IST), Windows will set the system clock to the same as the
wall clock, and Linux will set the system clock to one hour behind the
wall clock, leading to all sorts of difficulties.

There's a kernel option to cope with this situation -
CONFIG_APM_RTC_IS_GMT ("RTC stores time in GMT").  If you are flipping
back and forth between 'doze and Linux, you should set this option to
'N', which basically tells Linux to keep the system clock at local
time.  It'd be nicer if Microsoft fixed Windows, of course, in fact I
believe this is a fix scheduled for the next release of W2k, code-named
"Porcus Aerialus".

              Colm

-- 
Colm Buckley BA BF | NewWorld Commerce, 44 Westland Row, Dublin 2, Ireland
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When everything's coming your way, you're in the wrong lane.