def extend(self, delta):
        """
        Extends this subscription by the provided delta.

        :param delta: The timedelta by which to extend this subscription.
        :type delta: timedelta
        """

        if delta.total_seconds() < 0:
            raise ValueError("delta must be a positive timedelta.")

        if self.trial_end is not None and self.trial_end > timezone.now():
            period_end = self.trial_end
        else:
            period_end = self.current_period_end

        period_end += delta

        return self.update(proration_behavior="none", trial_end=period_end)

        Extends this subscription by the provided delta.

        :param delta: The timedelta by which to extend this subscription.
        :type delta: timedelta
        
djstripe/models/billing.py