diff --git a/sigal/writer.py b/sigal/writer.py
index 5356fe3..d2b566f 100644
--- a/sigal/writer.py
+++ b/sigal/writer.py
@@ -26,8 +26,10 @@ from __future__ import absolute_import
 import codecs
 import jinja2
 import logging
+import imp
 import os
 import sys
+import types
 
 from distutils.dir_util import copy_tree
 from jinja2 import Environment, FileSystemLoader, ChoiceLoader, PrefixLoader
@@ -81,6 +83,13 @@ class Writer(object):
             **env_options
         )
 
+        # handle optional filters.py
+        filters_py = os.path.join(self.theme, 'filters.py')
+        if os.path.exists(filters_py):
+            mod = imp.load_source('filters', filters_py)
+            for name in [name for name in dir(mod) if isinstance(getattr(mod, name), types.FunctionType)]:
+                env.filters[name] = getattr(mod, name)
+
         try:
             self.template = env.get_template(self.template_file)
         except TemplateNotFound: