You're right. There are two settings - "Daylight savings" and "Set time by GPS" (or something like that). The "Set time by GPS" option is on, but it was still an hour behind. I had to use the Daylight Savings option to get it right.
The only car clock I've ever had which has been right was an old Sony radio cassette player (those were the days!) I had in a Vauxhall Nova. One of the early ones with RDS. The time was always right. That was in 1995....