Comment 8 for bug 1237361

This happened to me after I added a "Time in other locations" (via the GUI: Settings > Time & Date > Clock) and at the same time, removed UTC.
I didn't want UTC to be shown, just my current time zone (British Summer Time) and the timezone in California, but it also showed UTC which is currently 1 hour behind (but won't be for much longer).

I fixed it with:
sudo dpkg-reconfigure tzdata
and a restart of the computer