Why is Diwali celebrated 21 days after Dussehra ?

Why is Diwali celebrated 21 days after Dussehra?. 

They told me that it took Lord Ram 21 days to walk from Sri Lanka to Ayodhya. I then checked it in Google maps and I was just shocked to see it does take 21 days (as per Google Walk) to get back.

So Lord Ram did exist and of course he knew the fastest route too!!! Jai Shree Ram