Prefer time zone or timezone?

[coke]

Pick one, add it to t/15*, and if “time zone”, probably have a few exceptions for attributes/method names.

When done, make sure aspell test passes.

[arkiuat]

Both are generally correct, so I vote for "timezone" so we don't have to fret about the necessary exceptions.

The only contexts in which a good case can be made for separating this compound word into the phrase that gave rise to it are non-technical contexts, which is definitely not what we're talking about here.