> So I'm dealing with a schema which has an attribute:
> 
>   <xs:simpleType  name="time">
>     <xs:union  memberTypes="
>           xs:gYear xs:gYearMonth xs:date xs:dateTime"  />
>   </xs:simpleType>
> 
> I had to write a function to let someone compare two of these 'time'
> simple types.  It appeared as though the only way to do 
> comparisions was with equal times, so that mean converting 
> everything to xs:dateTime.

For better or worse, the gXX types are very poorly supported by standard
functions and operators. I think the reason for this is that a lot of people
on the WGs felt that (a) these types were of very specialized interest
compared with other primitive types, and (b) date/time handling already had
far too big a share of the function library even without adding these to the
mix. 

It's also true that union types aren't all that well supported in the type
system. You can declare a function that takes as="element(*,time)" or
as="attribute(*,time)" as an argument, but you can't say as="time" because
it's not an atomic type.

So I think I would probably end up with something fairly similar to your
solution.

Michael Kay
http://www.saxonica.com/