Refactor Zim timestamp generator as a defun

zim.el

@@ -1,6 +1,19 @@
-(defun insert-zim-timestamp ()
-  (interactive)
+(defun zim-timestamp ()
+  (with-temp-buffer
     (insert (format-time-string "%Y-%m-%dT%H:%M:%S%z"))
     (forward-char -2)
     (insert ":")
-  (forward-char 2))
+    (forward-char 2)))
+
+(defun insert-zim-timestamp ()
+  (interactive)
+  (insert (zim-timestamp)))
+
+(defun insert-zim-header ()
+  (interactive)
+  (save-excursion
+    (goto-char (point-min))
+    (insert
+     (concat "Content-Type: text/x-zim-wiki\n"
+             "Wiki-Format: zim 0.4\n"
+             "Creation-Date: " (zim-timestamp) "\n\n"))))