The format for subject line date is !yyyy/mm/dd
I never remember this format especially since it is not a common format I see all over various apps on my Mac, so I never use this feature. I have to go into Evernote and manually add a date, which seems like a cumbersome work process.
It would be great of the date format could be enhanced to use the display format configured in Mac system preferences.
As long as I am wishing, it would also be handy to accept an optional time with the date format.
This idea is potentially applicable across many devices.