You're confusing time zone with UTC offset. The two are not the same concept. You can determine a UTC offset given a time zone ID and a date/time but the opposite is not necessarily true. A time zone ID cannot be reliably discovered given only its UTC offset; the mapping is not unique.
Yeah! We work around that in our logic at FastMail by choosing the largest population area which exactly matches the tzmapping data (generally it's from an icalendar file with some weird name and some rules which show the next couple of timezone transtions)
