(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