Date-time

- addresses parts of oasis-tcs#469
- specify . as frac-sec separator
- explicitly state no empty timezones
- add reference to ABNF of RFC 3339 section 5.6

csaf_2.1/prose/edit/src/design-considerations-02-date-time.md

@@ -5,7 +5,9 @@ In accordance with RFC 3339 and ISO 8601, the following rules apply:
 
 - The letter `T` separating the date and time SHALL be upper case.
 - The letter `Z` indicating the timezone UTC SHALL be upper case.
-- Fractions of seconds are allow as specified in the standards mention above.
+- Fractions of seconds are allow as specified in the standards mention above with the full stop (`.`) as separator.
 - Leap seconds are supported. However, they SHOULD be avoided if possible.
+- Empty timezones are prohibited.
+- The ABNF of RFC 3339, section 5.6 applies.
 
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