To get around the problems of manually setting the date on every system, a utility named rdate can be used. This will set the local system time to that of a specified remote machine. For example, to set helium to the same date as hydrogen we would run:
helium# rdate hydrogen Sun October 22 17:30:00 GMT 2002 hydrogen#
To automate this procedure, we could nominate a machine to be our “time server.” As explained above, we have chosen hydrogen for this task. First, we set the correct time on hydrogen. Next, we run rdate on a regular basis, by adding an entry to the root crontab. The redirection will ensure that we don't receive the output as mail every time cron runs:
helium# crontab -l | grep rdate 0 4,12,20 * * * rdate timeserver > /dev/null ...