換題日 ---間隔天數

01/03/2013 --- 28

01/31/2013 --- 28

03/02/2013 --- 30

04/06/2013 --- 35

05/06/2013 --- 30

06/06/2013 --- 31

07/13/2013 --- 37

08/16/2013 --- 34

09/12/2013 --- 27

10/11/2013 --- 29

11/08/2013 --- 28

12/08/2013 --- 30

平均天數 --- 30.6