It may also be a handset setting. I just set someone up with a Nokia 1600 and there were three choices related to setting date and time from the network: no, prompt, auto.
It was a brand new handset and this setting was set to "no" (so don't get date and time from network) — I had to enter date and time. I'd think "auto" would have it checking regularly and it would magically switch to DST.
-aw