TEI: Guidelines for Electronic Text Encoding and Interchange

If both when and dur are specified, the values should be interpreted as indicating a span of time by its starting time (or date) and duration. In order to represent a time range by a duration and its ending time the when-iso attribute must be used.

In providing a ‘regularized’ form, no claim is made that the form in the source text is incorrect; the regularized form is simply that chosen as the main form for purposes of unifying variant forms under a single heading.