MSXLS equivalent for xs:date('2010-11-20')+ xs:yearMo

Hello

I've to maintain an XSLT 1.0 stylesheet on .net framework.

I've found that on this version 

Cannot find the script or external object that implements prefix
'http://www.w3.org/2001/XMLSchema'.

so no xs:date and xs:yearMontDuration

Does anybody know an equivalent for

xs:date('2010-11-20')+ xs:yearMonthDuration( concat( 'P',$x,'D'))


I've to add $x days to a date. Googled, but hard to find.

Many thanks

Best regards.