I want to filter a document library by created date, I want to create a view from
today's date - 20 days only show in view.
normally for example Today date less than 20/09/2016 ?
You can create a dynamic view by setting it in the view configuration as:
Created is greater than or equal to [Today]-20
This will always show you files from the past 20 days.
The way it is worded though, you may want to switch it up to less than or equal to [Today]-20 if you want to see anything older than 20 days. I'm not entirely certian which way you want to go.