How to validate that end date is greater than start date

I have two date fields: Start date and End Date. I am trying to find a way to validate that the end date is greater than or equal to the start date. If I used datediff, it gives me the number of days difference between the two dates, but only as a positive number (e.g. 1/8/2015 & 4/8/2015 = 3; 4/8/2015 & 1/8/2015 = 3, so using this to catch where the second example would normally be -3!!

I have looked everywhere to try and find a simple solution....HELP!!!