Message
date must have an actual date tagged in year, month and day.
Rule XPath:
This rule ASSERTS the following, raising an alert if FALSE:
if (every $x in (year|month|day) satisfies $x castable as xs:integer) then string-join((year[string-length(.)
eq 4], format-number(month, '00'), format-number(day, '00')), '-') castable as xs:date
else false()