(defun foo-bar (bound)
  (let (beg end)
    (and (re-search-forward
	  "\\(\\(?:\\(?:Fri\\|Mon\\|S\\(?:atur\\|un\\)\\|\\(?:T\\(?:hur\\|ue\\)\\|Wedne\\)s\\)day\\)\
, \\(?:A\\(?:pril\\|ugust\\)\\|December\\|February\\|J\\(?:anuary\\|u\\(?:ly\\|ne\\)\\)\
\\|Ma\\(?:rch\\|y\\)\\|\\(?:Novem\\|Octo\\|Septem\\)ber\\) [0-9]+, -?[0-9]+\n\\)\\(=+\n\\)?" bound t)
	 (or (and (> (match-end 0) (match-end 1))
		  (setq beg (match-beginning 0))
		  (setq end (match-end 0)))
	     (and (= (match-end 1) bound)
		  (setq beg (match-beginning 0))
		  (save-excursion
		    (save-match-data
		      (and (looking-at "=+\n")
			   (setq end (match-end 0)))))))
	 (and beg end
	      (or (put-text-property beg end 'font-lock-multiline t) t)))))

(setq foo-font-lock-keywords
      '((foo-bar 1 'font-lock-keyword-face)))

(defun foo-mode-after-change (start end old-len)
  (save-excursion
    (goto-char start)
    (beginning-of-line)
    (when (eq (char-after) ?\=))
      (let* ((buffer-undo-list t)
	     (inhibit-read-only t)
	     (inhibit-point-motion-hooks t)
	     (inhibit-modification-hooks t)
	     deactivate-mark
	     buffer-file-name
	     buffer-file-truename)
	(put-text-property
	 (line-beginning-position 0) (line-beginning-position) 'fontified nil))))

(define-derived-mode foo-mode fundamental-mode
  "foo"
  (set (make-local-variable 'font-lock-defaults)
       '(foo-font-lock-keywords t))
  (set (make-local-variable 'font-lock-multiline) t)
  (add-hook 'after-change-functions 'foo-mode-after-change nil t))