Here is the formula to calculate the difference between two dates in Salesforce. The difference is returned in Days, Hours & Minutes. Works like a charm.

TEXT(
FLOOR(End_Date__c - Start_Date__c)
) & " Day(s) " &
TEXT(
ROUND(MOD((End_Date__c - Start_Date__c)*24,24),0)
) &" Hour(s) " &
TEXT(
ROUND(MOD((End_Date__c - Start_Date__c)*1440,60),0)
) &" Minute(s) "

“Formula

Reference: https://success.salesforce.com/answers#!/feedtype=SINGLE_QUESTION_DETAIL&dc=All&criteria=BESTANSWERS&id=90630000000gkp8AAA