String to Datetime(dd/mm/yyyy)

How to derived string to datetime (dd/mm/yyyy)

Input Date :

Start Date 

15/06/2015

30/06/2015

NULL

2015/06/24

Some date are in format dd/mm/yyyy I want to keep them as it is , but I want to convert the NULL one and 2015/06/24 to 24/06/2015.

How can I do this .