Set date for a blog item

Is there a way to set the date of a blog item in the item itself?
I've tried in the file item.md to set
date: 2016-08-01
in addition to the existing title: 'my title'
but all I get is DateTime::__construct(): Failed to parse time string (1470009600) at position 7 (6): Unexpected character in the admin page.

Ideas? Thanks

This is in the docs: headers / Frontmatter