> Is there a way to know:
> 
> 1. How many days are in a given month?

Try 

(xs:date('2006-xx-01') + xdt:yearMonthDuration('PT1M') -
xs:date('2006-xx-01') 
div xdt:dayTimeDuration('P1D')

> 2. How to add a duration to a given date (e.g. add a day to 
> '2006-03-31'
> to produce '2006-04-01')?

xs:date('2006-03-31') + xdt:dayTimeDuration('P1D')


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