Date-time

- addresses parts of oasis-tcs#469
- add section on Date and Time to explain the rules

csaf_2.1/prose/edit/src/design-considerations-01-construction-principles.md

@@ -66,4 +66,4 @@ Section [sec](#distributing-csaf-documents) states how to distribute and where t
 Safety, Security and Data Protection are considered in section [sec](#safety-security-and-data-protection-considerations).
 Finally, a set of conformance targets describes tools in the ecosystem.
 
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+

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

@@ -0,0 +1,11 @@
+## 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:
+
+- 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.
+- Leap seconds are supported. However, they SHOULD be avoided if possible.
+
+-------