Just to confuse matters, yes, PST is currently 8 hours behind GMT or UTC, but in california they are currently on PDT. In the UK, we're on BST (British Summer Time) which is one hour ahead of GMT/UTC, so California is currently 8 hours behind BST. If the downtime commences at 6pm GMT/UTC, then that will be 7pm BST and 11am PDT.
If anyone understood what I just said, could you sticky me and explain it to me, please :-).
If you want to know your current local time anywhere in the world, and compare it with GMT/UTC, see the World Clock [timeanddate.com]
Edit reason: I got myself confused between PST and PDT.