(Non)deterministic XML Schema

[Gijs]

I wish to allow any of the following five sequences to specify the
date of an historical event.

<EventDate>1677-06-21</EventDate>
or
<EventDate modifier="approximately">1677-06-01</EventDate>
or
<EventDate modifier="after">1677-01-01</EventDate>
or
<EventDate modifier="before">1678-01-01</EventDate>
or both
<EventDate modifier="after">1677-01-01</EventDate>
<EventDate modifier="before">1678-01-01</EventDate>

I guess that the solution will need derivations of an abstract type
and the use of the instance namespace, but I can't figure out an XML
Schema that is deterministic.

Who can help me?
Thanks and regards,
Gijs