MarkLogic / XQuery: dealing with date (duration) calculations

durations.xqy

xquery version "1.0-ml";

declare variable $dt as xs:dateTime := fn:current-dateTime();

element data {
  element current {$dt},
  element current-minus-seven-days {$dt - xs:dayTimeDuration('P7D')},
  element current-plus-seven-days {$dt + xs:dayTimeDuration('P7D')},
  element current-plus-one-hour {$dt + xs:dayTimeDuration('PT1H')}
}