because GetTimeZoneInformation() is slow. speeds up things like the recordings list.
store the timezone bias in a static, so we don't call GetTimeZoneInfo…
…rmation() on each conversion (it's slow)
reset the cached timezone bias value when the timezone changed on linux
Looks OK to me, assuming the threadsafety isn't an issue (doesn't look as though it is - at worse you'll get incorrect timezone data).
yeah thread safety isn't a problem here.