Created Date Format

I am having this problem with core "created" field date format.
I try to change it to custom "d/m/y" (as No. 1 on image) but when I save the field it returns back to "I, d F Y" (as No. 2 on image).

Before I do a "save and close" but just a save, on the articles it shows like No. 3 on image

When I save and close it shows like No. 4 on image.

How do i fix this?

I tried with a custom date field which shows well in article but when I include the customdatefield in prefix/suffix it shows wrong format (Y-m-d) so I tried with created field

but no luck.