[O] html preamble

[hymie!]

So my next task is to learn about the HTML preamble.

>From reading the docs, it looks like all of my org files have to share a
single HTML preamble, which is set in my .emacs file through either the
org-html-preamble variable, the org-html-preamble-format variable, or the
org-publish-project-alist variable.  There is no way to tell a specific Org
file "This is your HTML preamble."  Is that correct?

Why are there two different variables org-html-preamble-format and
org-html-preamble if the org-html-preamble variable can perform all of the
tasks of org-html-preamble-format and more?

Finally, how do I get the current date into the preamble?  I see that I can
specifically set a date, and I see the current date in a comment at the top
of my exported HTML, but I can't figure out how to get the current date out
of the comment and into my preamble.

Thanks.

--hymie!