Actually, Paul, you can make a pretty close estimate to your USDA Hardiness Zone by asking yourself "what's the coldest winter nighttime low we get, in a normal winter?"
You can at least bracket it by picking the low that you hardly EVER get lower than.
Then pick a low that you KNOW you reach in most years.
Your "Zone" is somewhere between those two.
That won't be a statically precise number, but it is the exact "spirit" of the hardiness number. Which is, as Carol implied, not very useful.
Here are the thresholds for some "Zones". If you never or almost never have frosts, you might be "Zone 10".
(click to expand)
