Adding a xs:yearMonthDuration to a xs:dateTime to obta

Hi all,
(In xslt2.0)I would like to increment a xs:dateTime
variable dt by one month. And have the result in (or
somehow converted to) xs:dateTime.
How can I achieve this.

Similar to using 
op:add-yearMonthDuration-to-dateTime( $arg1  as
xs:dateTime, $arg2  as xs:yearMonthDuration) as
xs:dateTime

but the above function may not be for direct use by
users as explained from at
[www.w3.org/TR/xpath-functions/#namespace-prefixes]
"These functions are not available directly to users,
and there is no requirement that implementations
should actually provide these functions. For this
reason, no namespace is associated with the op prefix.
"

I am using saxon8.
If I use 
<xsl:variable name="tmp3"
select='$currentGridDate+xs:yearMonthDuration("P1M")'/>
or
<xsl:variable name="tmp3" as="xs:dateTime"
select='$currentGridDate+xs:yearMonthDuration("P1M")'/>

The datetype of $tmp3 will be xs:dayTimeDuration in
both cases.

I would like to have the result in xs:dateTime so I
could call a template to recursively increment the
result by on month while making use of the result at
each recursion (variables are static final in xsl).

Allan.


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