Clock adjusting in SuSE Linux micro-howto

The system clock is set internally to UTC^[2]; when asked to display the time, it converts it to local time.

The CMOS on a Linux system should be UTC. However, we can set it also to localtime, specially if we double boot to some other OS. That is a special case that has to be considered on boot.

A digital clock is always precise: there can be no doubt about the time it says it is. Being precise is not the same as being accurate, however: the time it says to be can be incorrect. Some definitions apply; I'll paste here what I found on The Free On-line Dictionary of Computing (27 SEP 03):

The software clock might be inacurate, drift erratically, if the OS looses interrupts (note: I say might). However, every quartz based electronic clock may be inacurate, but it is always the same inacuracy: that is, it is always fast or slow by the same amount - or at least almost so. Therefore, if we know that amount by comparison to an external reference, we can make use of the fact that it is installed on a computer, and calculate and compensate for that constant drift.

That is what the program hwclock does, amongst other things, keeping the adjustment in /etc/adjtime.

Pity we can not do the same for our ordinary clock and watches.

For the purpose of this writeup, use only the command date on a console or terminal. Don't even look at desktop gadgets, because sometimes they could be wrong (buggy) and confuse us.

nimrodel:˜ # hwclock --show ; date ; date -u 
	  Thu Dec 11 03:54:53 2003  -0.885510 seconds
	  Thu Dec 11 03:54:41 CET 2003
	  Thu Dec 11 02:54:41 UTC 2003
	

Don't.

On a Linux (or Unix) system the clock should not be touched lightly. Things can misbehave (cron, for one. X, for another). You may affect other users, etc. However, if you need to set up the clock, continue reading.

The script /etc/init.d/boot.clock is responsible for setting up the clock during boot - read it for details. It takes some configuration from /etc/sysconfig/clock (or /etc/rc.config on older SuSE versions). I'll excerpt here the important parts.

If we have a s390 computer, and the CMOS clock is set to localtime, this is executed:

	if test "$HWCLOCK" != "-u"; then
	date $(date -u +'%m%d%H%M%Y')
	rc_status
	fi

Not too neat, perhaps a “hack”. After all, a Linux system should use UTC ;-)

Otherwise, for the rest of PCs, this is executed to set up the clock:

	echo -n Setting up the CMOS clock
	test -f /etc/adjtime || echo “0.0 0 0.0” > /etc/adjtime
	/sbin/hwclock --adjust $HWCLOCK
	rc_status
	/sbin/hwclock --hctosys $HWCLOCK

First, if the file /etc/adjtime does not exist, it is recreated with default valued (that's why if the adjust is going wrong, simply deleting that file works). Then, the hardware clock is adjusted (compensated) for its known constant drift. Finally, the system clock is set up from the hardware clock. Simple :-)

The script /etc/init.d/halt does many things, and one of them is:

	echo "Sending all processes the KILL signal..."
	killall -9
	echo -e "$rc_done_up"

	if test "$HOSTTYPE" = "s390" ; then
	HWCLOCK_ACCESS=no
	fi

	if test "$HWCLOCK_ACCESS" != "no" ; then
	if rc_active xntpd ; then

	echo -n "Set Hardware Clock to the current System Time"
	# write back to hardware clock and calculate adjtime
	/sbin/hwclock --systohc $HWCLOCK

	rc_status -v -r
	fi
	fi
	

What is that? Well, it checks to see if xntpd daemon was defined to run, and if so it assumes the system clock is correct, and therefore finally copies it back to the hardware (CMOS) clock, and calculates the drift to consider.

If we setup the system clock with date or a similar command we'll have problems, because the hardware clock is not synchronized. If we synchronize it incorrectly, the system is set up in such a way that it might think that is the constant drift it has to add on every boot, and the clock will be set up wrongly every boot up. There is a sequence of events that has to be respected.

This method can be used in cases where we don't have a permanent network connection, only a temporary one (or we don't want to run the daemon for whatever reasons). The sequence is:

For example, it can be inserted in /etc/ppp/ip-up.local script and it will run automatically every time we connect using the modem:

	SERVICE_NAME=ip-up.local
	NTPDATEOUT=/etc/ppp/ntpdate.output    

	/etc/init.d/xntpd ntptimeset &/bin/logger -t $SERVICE_NAME
	-p user.info " --> Synchronizing time">
	$NTPDATEOUT
	. /etc/sysconfig/clock
	/sbin/hwclock --systohc $HWCLOCK

	/bin/logger -t $SERVICE_NAME -p user.info -f $NTPDATEOUT
      

For this to run, we have to previously edit the file /etc/ntp.conf, where we only need to insert the time servers we'll use as references:

	## Outside source of synchronized time 
	## 
	server ntp.host.domain    # address of server 1
	server ntp.host2.domain   # address of server 2
      

Delete file /etc/adjtime after adjusting both system and hardware clock as explained above. Don't worry, it will be recreated on next boot.

Configure and start service rcxntpd - you can use Yast runlevel editor to enable this. I understand you can also use Yast to configure this service on SuSE 9.0, but certainly not on previous versions.

You will have to look elsewhere for information or details on configuring and running this service: a permanent network connection is needed, and I don't have one.

I can usually be found in the SuSE English and Spanish mail lists.

All the information in this document is believed to be correct - however, there may be errors. If I'm told, I'll correct them in the next version of this document.