Comment 42 for bug 104091

This problem is called "systematic time drift", it seems to happen a lot and it is explained in the manual page:
http://manpages.ubuntu.com/hwclock
http://bugs.archlinux.org/task/13684

[quote]
Automatic Hardware Clock Synchronization By the Kernel

You should be aware of another way that the Hardware Clock is kept
synchronized in some systems. The Linux kernel has a mode wherein it
copies the System Time to the Hardware Clock every 11 minutes.
This is a good mode to use when you are using something sophisticated
like ntp to keep your System Time synchronized. (ntp is a way to keep
your System Time synchronized either to a time server somewhere on the
network or to a radio clock hooked up to your system. See RFC 1305).

This mode (we'll call it "11 minute mode") is off until something
turns it on. The ntp daemon xntpd is one thing that turns it on. You
can turn it off by running anything, including
hwclock --hctosys,
that sets the System Time the old fashioned way.

To see if it is on or
off, use the command
adjtimex --print
and look at the value of "status". If the "64" bit of this number
(expressed in binary) equal to 0, 11 minute mode is on. Otherwise, it
is off.

If your system runs with 11 minute mode on, don't use
hwclock --adjust
or
hwclock --hctosys.
You'll just make a mess. It is acceptable to use a
hwclock --hctosys
at startup time to get a reasonable System Time until your system is
able to set the System Time from the external source and start 11
minute mode.
[/quote]