tlug: time

["Stephen J. Turnbull" <turnbull@example.com>]

>>>>> "John" == John Seebach <jseebach@example.com> writes:

    John> The hardware clock in my laptop has an annoying habit of
    John> losing time from, uh, time to time.

How much time?  And does it happen while you're running, or only when
you have a suspend/crash?

BTW, I know someone else with a notebook that does this, but I forget
the make/model.  (Steve Baur, you can buttonhole him at the next TLUG
meeting at lunch or after his speech.)  I think he just lives with it;
it's a client terminal, not a server host, and he has a keitai to get
the clock time from.  ;-)

    John> How can one safely change the time of a running system?

Running?  Or booting?

    John> I'm sure there's a way, but I haven't been able to find it. 

In general I think you're probably wrong; no matter what you do to
change the time, there's probably an app out there you can screw up
by doing it.

But at boot time, there are two main issues I can think of: file
date/clock skew, and cron running multiple times on wake up.  As long
as you are not losing a lot more time on the hw clock than on the
system clock, the boot process, including fsck, is not going to cause
skew.  So if you set up your system to not start cron in single-user
mode, and boot to single-user, then reset the clock, then telinit to
your usual runlevel, you should be OK.

AFAIK there's no difference between `date -s ...; hwclock --systohc',
and `hwclock --set --date=...; hwclock --hctosys', except if your
system crashes on the semicolon.

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