# On Linux systems with logrotate,
#  drop this into /etc/logrotate.d/ntpd
# That will rotate ntpd.log monthly and then
# kick ntpd to switch to the new log file.

/var/log/ntpd.log {
    monthly
    missingok
    postrotate
      /usr/bin/killall -HUP ntpd
    endscript
    rotate 6
}