converting a bson datetime to an elisp time

gistfile1.el

(defun bson-datetime-int64-to-time (byte-list)
  (let ((calc-num
         (concat
          "16#"
          (mapconcat
           (lambda (x) (format "%02X" x))
           byte-list ""))))
    (list
     (calc-eval
      "rsh(and(idiv($,1000),16#ffff0000),16)"
      'rawnum
      calc-num)
     (calc-eval
      "and(idiv($,1000),16#ffff)"
      'rawnum
      calc-num))))

(format-time-string
 "%D"
 (bson-datetime-int64-to-time
  (reverse (list #x40 #xa0 #x9f #xc3 #x2c #x01 #x00 #x00))))