How to calculate difference of two dates in numeric values

Question

Hai ,

 

In my project I need to find total running days of  from the order date to current date or Today date ,

How can I do this in my project .

Please suggest any idea to finish this concept and it's urgent please somebody help me to solve this. 

 

For an example :

 

Order Date :01-10-2015   and  Today Date:13-10-2015  

 

Order running Days : 13 

 

 This is my concept. I hope you can understand.