This element is typically used for dates for which months and years are not given (for example, “In apple blossom time”). While the <string-date> element may also be used for any date that is represented as a sequence of characters (i.e., “January, 2001”, “Fall 2001”, “March 11, 2001”), it is best practice, when possible, to tag the year, month, and/or day as numbers, so “January, 2001” would become:

...
<month>01</month>
<year iso-8601-date="2001-01">2001</year>
...

This Tag Set allows <string-date> both inside <date> and at the same level as <date>. This flexibility enables preservation of nearly any publisher’s structure.