Date-time

- addresses parts of oasis-tcs#469
- add links to standards
- add rules regarding separator

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

@@ -1,13 +1,14 @@
 ## Date and Time
 
 This standard uses the `date-time` format as defined in JSON Schema Draft 2020-12 Section 7.3.1.
-In accordance with RFC 3339 and ISO 8601, the following rules apply:
+In accordance with [cite]{#RFC3339} and [cite]{#ISO8601}, the following rules apply:
 
 * The letter `T` separating the date and time SHALL be upper case.
+* The separator between date and time MUST be the letter `T`.
 * The letter `Z` indicating the timezone UTC SHALL be upper case.
 * Fractions of seconds are allowed 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.
+* Empty timezones MUST NOT be used.
 * The ABNF of RFC 3339, section 5.6 applies.
 
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