- date_create: documentation ( source)
- number_format: documentation ( source)
<?php
$data['PremiumStartTime'] = '2018-12-12 21:13:00';
$data['PremiumDays'] = 20;
$days_remaining = $data['PremiumDays'] - date_create($data['PremiumStartTime'])->diff(date_create())->days;
echo number_format($days_remaining);