Xquery Xpath Date Calculation in c# Using SAXON

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C#

XPath

i want output from below Expiration
xs:date($refPeriodEndDate)-xs:dayTimeDuration('P14D')
i want to pass date as parameter and get output of Expiration.

i have used "Saxon.Api" but got error - Invalid date "xs:date($refPeriodEndDate)-xs:..." (Year is less than four digits)

if any one used Saxon and have any function to Calculation it. help me..