Design for extensions/timezone data

DEVELOPMENT.md

@@ -5,7 +5,7 @@
 An implicit interval, such as `2022` is equalivent to the explicit interval `2022-01/12`. The transformation proceeds as follows:
 
 1. Enumeration for implicit intervals occurs at the next hightest zoom level (greater resolution). In this case is is months.
-2. The end of the interval is a closed at month 12. 
+2. The end of the interval is a closed at month 12.
 3. Therefore we can say the implicit interval is closed and equivalent to the set `{2022-01, 2022-02, ..., 2022-12}`
 
 ### On conversion between explicit and implicit intervals
@@ -19,4 +19,53 @@ An implicit interval, such as `2022` is equalivent to the explicit interval `202
 * Parsing should extend the end of the interal with the units of lower resolution in the beginning of the interval
 * Both parts of the interval should have the same resolution. When they are different, what is the treatment????  Looks like the standard says that the interval end inherits the higher resolution time units from the interval start.  But what if the interval end has higher resolution?
 
+### Timezones
+
+A [proposed extension](https://datatracker.ietf.org/doc/draft-ietf-sedate-datetime-extended/) to [rfc3339](https://www.rfc-editor.org/rfc/rfc3339) specifies a mechanism to add metadata, including timezones, to a RFC3339 datetime.  RFC3339 is a profile of ISO8601 that is largely the "extended" syntax of ISO8601 limted to the capability of Part 1.  Therefore ISO8601 is a superset of RFC3339.
+
+The ABNF of the extension mechanism is described as follows and will be used for implementation in Tempo.  Since a timezone offset and a timezone extension name may be in conflict, the "Critical" flag is used to raise an exception if the explicit offset and the implcit offset (of the timezone name) do not match.
+
+If the Critical flag is not provided, then the policy will be that the time zone name will take precedence. The extension proposal explicity calls out that the policy to apply is implementation dependent, hence the clarity required here.
+
+### Extensions
+
+The [RFC3339 proposed extensions mechanism](https://datatracker.ietf.org/doc/draft-ietf-sedate-datetime-extended/) also allows additional metadata to be attached to a date time. The key `u-ca` is based upon BCP47 "U" extension tag and is therefore aligned with the [CLDR locale extension](http://www.unicode.org/reports/tr35/#Locale_Extension_Key_and_Type_Data).
+
+For Tempo we extend this somewhat in the following way:
+
+1. The entire BCP47 "U" syntax can be supplied as an extension. It can therefore convey more than just the calendar (for example, number system).
+2. The 'u-ca`  calendar must follow the CLDR calendar syntax which cannot represent the arbitrary calendars that can be defined in Elixir.
+3. Therefore we introduce an additional suffix key `calendar` which can carry any Elixir module name as a string that represents an Elixir calendar representation.
+
+### Relationship to ex_cldr
+
+In the case where no calendar is specified by an extension, the calendar specified by `Cldr.get_locale/0` will be used. This is most commonly `Cldr.Calendar.Gregorian` which is a proleptic Gregorian calendar that is compatible with the Elixir standard `Calendar.ISO`.
+
+### Extension ABNF syntax
+
+```
+time-zone-initial = ALPHA / "." / "_"
+time-zone-char    = time-zone-initial / DIGIT / "-" / "+"
+time-zone-part    = time-zone-initial *13(time-zone-char)
+                   ; but not "." or ".."
+time-zone-name    = time-zone-part *("/" time-zone-part)
+time-zone         = "[" critical-flag
+                       time-zone-name / time-numoffset "]"
+
+key-initial       = ALPHA / "_"
+key-char          = key-initial / DIGIT / "-"
+suffix-key        = key-initial *key-char
+
+suffix-value      = 1*alphanum
+suffix-values     = suffix-value *("-" suffix-value)
+suffix-tag        = "[" critical-flag
+                       suffix-key "=" suffix-values "]"
+suffix            = [time-zone] *suffix-tag
+
+date-time-ext     = date-time suffix
+
+critical-flag     = [ "!" ]
+
+alphanum          = ALPHA / DIGIT
+```