Return the expire time passed to the templatetag.
        Must be None or an integer.

def get_expire_time(self):
        """
        Return the expire time passed to the templatetag.
        Must be None or an integer.
        """
        try:
            expire_time = self.node.expire_time.resolve(self.context)
        except template.VariableDoesNotExist:
            raise template.TemplateSyntaxError('"%s" tag got an unknown variable: %r' %
                                               (self.node.nodename, self.node.expire_time.var))
        try:
            if expire_time is not None:
                expire_time = str(expire_time)
                if not expire_time.isdigit():
                    raise TypeError
                expire_time = int(expire_time)
        except (ValueError, TypeError):
            raise template.TemplateSyntaxError(
                '"%s" tag got a non-integer (or None) timeout value: %r' % (
                    self.node.nodename, expire_time
                )
            )

        return expire_time