post: Add post-date procedure.

* haunt/post.scm (%default-date): New variable.
  (post-date): New procedure.
* haunt/builder/atom.scm (post->atom-entry): Use post-date.
* README.md (Example Configuration): Remove date from default metadata.

haunt/builder/atom.scm

@@ -51,7 +51,7 @@
      (name ,(post-ref post 'author))
      ,(let ((email (post-ref post 'email)))
         (if email `(email ,email) '())))
-    (updated ,(date->string* (post-ref post 'date)))
+    (updated ,(date->string* (post-date post)))
     (link (@ (href ,(string-append "/" (post-slug post) ".html"))
              (rel "alternate")))
     (summary (@ (type "html"))

haunt/post.scm

@@ -34,6 +34,8 @@
             post-metadata
             post-ref
             post-slug
+            %default-date
+            post-date
             posts/reverse-chronological
             posts/group-by-tag
 
@@ -59,6 +61,14 @@
                                   char-set:whitespace))
                "-"))
 
+(define %default-date
+  (make-date 0 0 0 0 1 1 1970 0)) ; UNIX epoch
+
+(define (post-date post)
+  "Return the date for POST, or '%default-date' if no date is
+specified."
+  (or (post-ref post 'date) %default-date))
+
 (define (post-time post)
   (date->time-utc (post-ref post 'date)))