A set of colon delimited codes that control the execution of specific valdation rules for date(/time) elements. The code tz-for-time controls whether the rule that there must be a timezone if there's a time is enforced - in it's absence, the rule is not enforced. The code year-valid controls whether the year is checked for being in the range 1800 -> now + 80 year - in it's absence, the rule is not enforced. If the extension is not present, the rules are enforced. This extension has no effect on data types and resources, only in logical models