As others have said, it's #[InjectDate()]#
, where the arguments are from the PHP's DateTime::format() method.
For a string like February 28th
you can use:
#[InjectDate('F jS')]#
You simply write it as part of the title, like this: Hello, today is #[InjectDate('F jS')]#
which right now would say Hello, today is May 7th
.
Note that it only woks in the title, not the post body.
You will see the preview of the title if it was posted right now under the title box so you can be sure it's correct.