Minor Description
The time zone string "EST" is parsed by calendarPattern charater "z", and becomes ISO standard "-05:00" on parse, but "GMT-5" on unparse.
This happens despite dfdl:calendarLanguge="en_US". That is, the problem is that the unparse should produce "EST", but does not.
The locale is needed to implement "z" time zone format because some of these 3-letter timezone specifiers are ambiguous, so the locale must be known to disambiguate them when parsing.
For example CST can be "Central Standard Time" (North America), "Cuba Standard Time" or "China Standard Time", and is widely used for "Central Standard Time" (Australia) according to https://en.wikipedia.org/wiki/List_of_time_zone_abbreviations
So when parsing "z" one must use the locale. When unparsing it's not so ambiguous, but Daffodil doesn't appear to use the locale information.
Per the ICU documentation here: https://icu.unicode.org/design/formatting/timezone/icu-4-8-time-zone-names
The "z" needs locale information, and several of the other specifiers also need it.