No, and this is deliberate rather than a bug. A trial delay (trial_days) determines the subscription's start date by itself; a day/month preference is a separate, incompatible way of pinning the start date. The plugin's form builder hides the day/month fields once a trial is set, and the subscription-creation code explicitly skips day/month resolution whenever a trial is active — sending both would give GoCardless two disagreeing signals about when the subscription should start.