Using "Ymd" under "Custom Input Format" is giving an error "The year is invalid"

I've created a Date field with "Ymd" as the value in the "Custom Input Format" under "More Settings and Values" of the Date field. Everything worked well till here, but when I tried adding the date into the desired format which is nothing but "Ymd" which is "20150509", it gave me an error stating that: "The value input for field Date is invalid: The year is invalid."

This should not happen since the I've provided the date in the exact date format which I've defined in the "Custom Input Format" under "More Settings and Values" of the Date field.

Attached are the screenshots for the same.