> -----Original Message----- > From: centos-bounces at centos.org [mailto:centos-bounces at centos.org] On > Behalf Of Kenneth Porter > Sent: Friday, March 04, 2011 14:15 > To: CentOS mailing list > Subject: [CentOS] Updating hardware clock from cron > > Is there a package to do this? > > Normally the hardware clock is set during shutdown if one is running > ntpd. No, hwclock --systohc is only called at start time (in /etc/rc.d/init.d/ntpd), and only if ntpdate got a good time, which is a good thing. > But if a long-running server shuts down unexpectedly, this isn't done, > and > the hardware clock might be off by a lot when it comes back up. Not if you are running ntp and it was able to sync, because ntpd activates a mode in the kernel that sets the hwclock every 11 minutes when ntp declares it got synced. If your hwclock is off by a lot when it comes up I believe it is from one of the following: A) bad cmos battery. B) poor cmos clock C) confusing info in /etc/adjtime due to using both hwclock --adjust [at boot] and ntp (long story, but it is due to both tweaking the clock without coordination between them). D) booting a different OS with different ideas of timezones. E) manual tweaking of time via bios. > So setting > it periodically from a cron job could be useful. > > What do others do? Adding a one liner to /etc/cron.daily that invokes > /etc/rc.d/init.d/ntpd would do it but it seems heavyweight to restart > ntpd. > Alas, the script doesn't export just the sync_hwclock function. Recommendation, Understand `hwclock --systohc` should _only_ be called when the admin knows a good system time was *_JUST_* set from a good source, i.e., following a successful call to ntpdate or the admin setting the systime with date. On some systems that do NOT have ntpd service available (not on a network with a time server), I will have them do a sequence of hwclock --hctosys hwclock --adjust hwclock --hctosys in /etc/rc.d/rc.sysinit where it already does the hwclock --hctosys so that once I have set the time from a known source a few times the box will reasonably self correct time on boot, I will even on some of those systems have that sequence in a cronjob that gets run once a week. If this is on a network, I have the box that is doing this serve ntp (via local clock) to the rest of the network, so they drift together. Reminder: `hwclock --systohc` should _only_ be called when the admin knows system time was JUST set from a good source, i.e., NOT from a cron job that is not also making sure the ntpdate worked.