Automatic Update NTP At Boot Time

Resetting the clock at boot time with ntpdate

The system clock is initialized from the hardware clock by the /etc/init.d/boot.clock script. (At least this is true in SuSE 8.1 and later; when booting Red Hat Linux 6.x, the system clock is initialized by /etc/rc.d/rc.sysinit instead.) Unfortunately, when this runs, the network has not yet been fully initialized, so it is not possible to query servers. Instead, it is better to run ntpdate from the /etc/init.d/boot.local script (called /etc/rc.d/rc.local in older versions), which is run as the very last thing when booting up (in run levels 2, 3, and 5, which is appropriate). Setting the system clock twice like this may leave it off by a bit during boot, but the amount by which it is off can be limited for the next boot by resetting the hardware clock after initializing the system clock from the NTP servers. You can do all of this by adding the following two lines to the end of your /etc/init.d/boot.local script (after replacing the server names):

    ntpdate -sb server1 [server2 ...]
    hwclock --systohc

The next time you boot, the hardware clock will only be off by the amount of drift between boots. (You should first check to see that you have hwclock on your system.)
The "-b" option forces the system clock to be set in one jump, rather than attempting to slew it gradually, and is recommended by the ntpdate documentation page when booting.
Note that this double clock setting procedure is essentially equivalent to what the standard boot-time startup scripts for ntpd do, so making the clock jump back and forth at boot time can't be all that bad. (It can make the log files harder to decipher, though.)
Unfortunately, the system boot scripts are very vendor-dependent, so this recipe may not work for your configuration. If there is an /etc/init.d/rc.local 2or /etc/init.d/boot.local script, it probably works the same way; otherwise, you will need to figure out something different for your flavor of Linux/*BSD/etc.