declare function local:addZero($int as xs:int) as xs:string
{
  let $string := xs:string($int)
  return if (string-length($string) = 2)
         then $string
         else string-join(('0',$string),'') 
}

declare function local:addDays($date as xs:date, $days as xs:int) as xs:date
{
  let $year := fn:year-from-date($date),
      $longYear := $year mod 4 = 0 and not($year mod 100 = 0 and not($year mod 400 = 0)), 
      $month := fn:month-from-date($date),
      $day := fn:day-from-date($date),
      $daysInMonth := if ($month = (1,3,5,7,8,10,12))
                      then 31
                      else if ($month = 2)
                           then if ($longYear) then 29 else 28
                           else 30, 
      $daysLeftInMonth := $daysInMonth - $day,
      $newDay := if ($days >= $daysLeftInMonth)
                 then $day + $days - $daysInMonth
                 else $day + $days,
      $newMonth := if ($days >= $daysLeftInMonth)
                   then if ($month = 12) 
                        then 1
                        else $month + 1
                   else $month,
      $newYear := if ($days >= $daysLeftInMonth and $month = 12)
                  then $year + 1
                  else $year
  return (xs:date(string-join((string($newYear),local:addZero($newMonth),local:addZero($newDay)),'-')))
}

local:addDays(current-date(),10)