After looking around, the declaration to use today should be this one:
<time itemprop="dateCreated" datetime="2018-03-02">
So in the <head> section of the page’s custom code we could do this:
http://vincent.polenordstudio.fr/snap/sel6i.jpg
Except that, as Sam said, it won’t work because we won’t be able to use the correct date format.
So if you want a proper workaround, you’ll need to add a field to your collection and enter a date in the proper format manually.
For the static pages, simply add the date in the page header code manually.