Here's how I visualize the relationship between the Pacific time zone and
UT. This has worked for me, but other peoples' mileage may vary. If this
sounds a little pedandtic, it's because it's a partial regurgitation of the
way I explain longitude in class...
Being east of us, Greenwich encounters a new day first, and so it must be
some number of hours ahead of us: 8, to be specific. So, to change from PST
to UT, we just move the clock ahead 8 hours, and accept any change of date
that may occur as a result. To change from UT to PST, we would set a clock
back 8 hours.
Here's the part that I had to work on for a while: Switching to daylight
savings time means we set the clock ahead one hour - it's like we've moved
ourselves to a location on earth that's ahead of California, i.e. east of
California. I think of DST as, essentially, setting our clocks to MST,
which is one hour ahead.
And the distance between the MST zone and Greenwich is smaller than the
distance between the PST zone and Greenwich, so the size of the "correction
factor" must be smaller. Hence a 7 hour difference instead of an 8 hour
one. We do the conversions the same way as above, but with 7 hours instead
of 8 hours.
This is my attempt to think about sign conventions as little as possible -
they have always tripped me up. I always try to think about it visually.
--- Marek Cichanski