In the spec of format-time(), the picture "[ZN]" indicates use of a timezone name such as PST. Presumably the country code and language code can be used to decide among the many different names for a given timezone offset (see http://www.timeanddate.com/library/abbreviations/timezones/). However, this doesn't solve the problem of summer time (daylight savings time). Given language "en" and country "US", a timezone of -07:00 can mean MST or PDT. One could resolve it, at least for dates in the past, using a perpetual calendar of dates on which the clocks moved forwards or backwards, but that doesn't seem a reasonable burden on implementations.

I would suggest that in the absence of a more coherent approach to our handling of timezones as distinct from time offsets, we remove this feature. I don't think it's implementable in an interoperable way.