Modify getTrialDays() to make all subscriptions start on the 10th of each month

Subscriptions.php

<?php

    protected function getTrialDays($details)
    {
        switch ($details['trial_period_unit']) {
            case null:
                $days = 0;
                break;
            case 'day':
                $days = $details['trial_period_frequency'];
                break;
            case 'week':
                $days = $details['trial_period_frequency'] * 7;
                break;
            case 'semi_month':
                $days = $details['trial_period_frequency'] * 14;
                break;
            case 'month':
                $days = $details['trial_period_frequency'] * 30;
                break;
            case 'year':
                $days = $details['trial_period_frequency'] * 356;
                break;
        }

        // If set to month, bill on the 10th
        if ($details['period_unit'] == 'month')
        {
            $day = date("j");

            if ($day < 10)
            {
                $days = 10 - $day;
            }
            else if ($day > 10)
            {
                $target = strtotime("first day of next month");
                $target = strtotime(date("c", $target) . " + 9 days");
                $datediff = $target - time();
                $days = floor($datediff/(60*60*24));
            }
        }

        return round($days);
    }