We have just changed back to winter time, and I am sure that there is a way to make the phone change, without setting the time. I can only find time zones, and have set it to GMT (which is my local zone), but even so, I am sure that there is another menu item somewhere to change to and from summer time. The phone time did not change, so it was already on the right time zone.
I've looked in the user guide, but there is no mention of it.
I can't remember what I did 6 months ago.