daytimeduration help

Using saxon 8.6.1 I get the error:
XPST0003: xpath syntax error...cannot find one-argument funcion 
in{http://www.w3.org/2005/04/xpath - datatypes}dayTimeDuration()
I am trying to add 8 hours to a time ie:
xs:dateTime('2005-01-10T22:05:00') + xdt:dayTimeDuration('PT8H') 

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