Creating a new reference column via a flat file (Date field)

Hi

I'm looking for some help with the below data. I want to create a new reference column - which is taking the date in the 'due date' column in the attached

For example if the due date is 30/11/2015 = i want to create a new refrence column that takes the year '2015' mutiple that times 12 then add the month - in this example 11 = 24191.

Is this possible? 

customer,docval,due-date
11926,648984,30/11/2015
11926,903787,30/11/2015
11926,976124,31/12/2015
11926,569599,31/12/2015
14340,829122,30/04/2015
14639,362986,30/04/2020
14639,374992,31/05/2020
14776,943918,30/05/2014
14776,718274,31/07/2014
14776,561896,31/10/2014
14776,458269,30/09/2015

Any suggestion or help would be greatly appreciated