Comment 7 for bug 1278359

If ntpd is running, ntpdate should never run - doing so causes clock jumps. In Trusty, /etc/network/if-up.d/ntpdate actually stops ntpd so ntpdate can run. Debian jessie doesn't do this, so if this was from upstream, it's been fixed.

From /etc/network/if-up.d/ntpdate:
-----
if [ -e /usr/sbin/openntpd ]; then
    service='openntpd'
else
    service='ntp'
fi

invoke-rc.d --quiet $service stop >/dev/null 2>&1 || true

/usr/sbin/ntpdate-debian -s $OPTS 2>/dev/null || :

invoke-rc.d --quiet $service start >/dev/null 2>&1 || true
------

Workaround:
sudo mkdir /etc/network/if-up.d.disabled
sudo dpkg-divert --divert /etc/network/if-up.d.disabled/ntpdate --rename /etc/network/if-up.d/ntpdate