[xsl] date calculations

[Geert Bormans <geert@xxxxxxxxxxxxxxxxxxx>]

Hi there,

I need some date calculations.

Here is my XML
<StartTime>2007-03-04T04:00:00Z</StartTime>
            <Duration>PT03H00M00S</Duration>

and I need to get this into
<StartTime>2007-03-04T04:00:00Z</StartTime>
            <EndTime>2007-03-04T07:00:00Z</EndTime>

I am using XSLT2, Saxon.
Is there enough functionality in the XPath date and duration functions,
to make the calculations (I can't seem to find something like endtime 
= starttime + duration).
If I need to make my own function, has someone already done this before?

Thanks

Geert