XQuery: Unix timestamp from dateTime

dateTime-to-timestamp.xq

(:~
 : unix time from dateTime
 : @param dateTime
 : @return unix timestamp of the supplied datetime
 :)
declare local:dateTime-to-timestamp(
    $dateTime as xs:dateTime
) as xs:integer {
    let $dayZero := xs:dateTime('1970-01-01T00:00:00-00:00')
    return
      ($dateTime - $dayZero) div xs:dayTimeDuration('PT1S')
}

()