Template tag and filter for displaying number of years since

from django import template

# helper closure

def do_parse1arg(nodeCLS):
    """Parse tag name and 1 argument, return nodeCLS node."""
    def _do_parse1(parser, token):
        try:
            # split_contents() knows not to split quoted strings.
            tag_name, arg1 = token.split_contents()
        except ValueError:
            raise template.TemplateSyntaxError("%r tag requires 1 argument" % token.contents.split()[0])
        return nodeCLS(arg1)
    return _do_parse1

def since(start_year):
    from datetime import date
    return str(date.today().year - int(start_year))

class YearsSinceNode(template.Node):
    """Renders number of years since a specific year."""
    def __init__(self, start_year):
        self.years = since(start_year)
    def render(self, context):
        return self.years
register.tag('years_since', do_parse1arg(YearsSinceNode))
register.filter('since', since)