The following code has been tested with a datepicker field but would work on any field formatted as a date. Place it in your custom plugin or functions.php file.

add_filter('gf_echeck_subscription_pre_create', 'az_change_subscription', 10, 4);

function az_change_subscription($subscription, $submission_data, $feed, $form) {
// change these values to match your form id and field id
$form_id = 22;
$field_id = 10;
if ($form['id'] == $form_id && isset($_POST['input_' . $field_id]) && !empty($_POST['input_' . $field_id])) {
     $subscription->startDate = date('Y-m-d', strtotime(sanitize_text_field($_POST['input_' . $field_id]))); 
}
return $subscription;
}