Working with xs:date and xs:gYearMonth

I'm trying to write a transform for generating calendards from some 
xml event data.

I shall want to highlight the entry for today (if the calendar happens
to be displaying the current month), so I set up a global variable
named today:


   <xsl:variable name="today" select="current-date()"  as="xs:date"/>
   
I also want to be able to pass a parameter to name the month to
display, defaulting to the current month, so:

     <xsl:param name="date" select="xs:gYearMonth($today)"  as="xs:gYearMonth"/>

As I want to generate a hyperlink to the previous month, I want a
variable that I can put into the query string of a URL so:

     <xsl:variable name="previous_month"
        select="xs:gYearMonth(xs:date($date) -
        xdt:yearMonthDuration('P0Y1M'))" as="xs:gYearMonth"/>

But when I try to reference this variable:

<th id="lastmonth"><a href="/onevoice/members/calendar?date='{$previous_month}'">&#171;</a></th>

saxon (8.0b) gives me:

Error at attribute on line 60 of file:/home/colin/onevoice/members/calendar.xsl:
  Cannot convert gYearMonth to xs:date
Transformation failed: Run-time errors were reported

How am I supposed to go about this?
-- 
Colin Paul Adams
Preston Lancashire