Okay, I didn't really need to create a new filter

lib/jekyll/filters.rb

@@ -1,18 +1,14 @@
 module Jekyll
   
   module Filters
-    def date_to_string(date)
-      date.strftime("%d %b %Y")
+    def date_to_string(date, format="%d %b %Y")
+      date.strftime(format)
     end
 
     def date_to_long_string(date)
       date.strftime("%d %B %Y")
     end
     
-    def strftime(date, format = '%x')
-        date.strftime(format)
-    end
-    
     def date_to_xmlschema(date)
       date.xmlschema
     end