declare function local:milliseconds-since-epoch($dateTime as xs:dateTime) as xs:integer
{
  let $epoch := xs:dateTime("1970-01-01T00:00:00Z")
  let $duration := xs:duration($dateTime - $epoch)
  let $days := fn:days-from-duration($duration) * 8.64e+7
  let $hours := fn:hours-from-duration($duration) * 3.6e+6
  let $minutes := fn:minutes-from-duration($duration) * 60000
  let $seconds := fn:seconds-from-duration($duration) * 1000
  return xs:integer($days + $hours + $minutes + $seconds)
};

local:milliseconds-since-epoch(fn:current-dateTime())