http://spodaryk.pp.ua/post/21/django-static-files-versioning

app_tags.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import os

from django import template
from django.conf import settings

register = template.Library()

# ...
# other template tags
# ...

@register.simple_tag
def static_version(path):
    """ Returns absolute URL to static file with versioning.
    """
    full_path = os.path.join(os.path.abspath(settings.STATICFILES_DIRS[0]), os.path.normpath(path))
    try:
        # Get file modification time.
        mtime = os.path.getmtime(full_path)
        return '%s%s?%s' % (settings.STATIC_URL, path, mtime)
    except OSError:
        # Returns normal url if this file was not found in filesystem.
        return '%s%s' % (settings.STATIC_URL, path)

base.html

...
{% load app_tags %}
...
<link rel="icon" href="{% static_version 'img/favicon.png' %}" type="image/x-icon">
<link rel="shortcut icon" href="{% static_version 'img/favicon.png' %}" type="image/x-icon">

<link rel="stylesheet" href="{% static_version 'css/main.css' %}" type="text/css" />
<script type='text/javascript' language="javascript" src="{% static_version 'js/main.js' %}"></script>
...

settings.py

INSTALLED_APPS = (
  # ...
  '<app_name>.app_tags',
  # ...
)