According to https://www.drupal.org/node/1455576,

As a fallback the widget will try to accept input allowed by the php strtotime function. This allows the user to type a date in in many natural formats, like 'March 31, 1980'.

However, this is not the case. If you try to enter a value that strtotime would understand, you get errors like "The value next thursday does not match the expected format," or "The value +2 hours does not match the expected format."

I'd like to know are the date fields supposed to fall back to strtotime, or is the documentation incorrect?