Why is my EC2 server's time off by ~10 seconds every day?

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http://unix.stackexchange.com – I have what I believe is a system file, /etc/cron.daily/ntpupdate which runs ntpdate ntp.ubuntu.com daily to sync with the network time. Every day it generates output very similar to this: /etc/cron.daily/ntpupdate: 16 Jan 06:30:42 ntpdate[21446]: step time server offset -12.646804 sec I'm not positive what the means but I'm pretty sure -12.646804 sec means that my server is off by around 12 seconds. But I don't know why it is off by around the same amount every day. This is an Amazon EC2 instance running Ubuntu. I can only guess that either it is losing / gaining (HowTos)