<date normalised="1993-05-01T15:30:00">May 1, 1993</date>
i.e. the user can put the date/time to be displayed in any desired form,
but a normalised form is added as an optional attribute. (The normalised
date here is not in RFC 822 form but the idea's the same).
One problem with this, however, is that it needs its own tag and cannot
be a logical role of the <em> tag. (This problem may arise for other
elements which are currently coded as role attributes.)
Steve.