From the main commit:
The gnome-desktop timezone monitor currently does not notify us about DST changes in the local timezone, but even if it would, that would not catch DST changes in the timezones of the world clock entries. Instead of trying to monitor for all possibly involved DST changes, simply always update the offset label whenever we update the time.
The two other commits are there to reduce the impact of calculating the offset every time the clock is updated. Not sure if they are really necessary though.