比较php中的日期并显示结果

我的网站上有以下功能:

function Active()
{
    ............

$num_rows = $db->doQuery('SELECT PremiumDays, PremiumStartTime FROM Premium WHERE AccountID = ?', $_SESSION['AccountID']);
if ($num_rows == -1)
{
$this->Error('ERROR');
$db->getError();
return;
}

$data = $db->doRead();
$data['Status'] = $num_rows == 0 ? '<:SHOW_PREMIUM_STATUS:>' : '<b><font size="2" color="red">Premium is active - <%Days_Remaining%> days remaining.</font></b>';

$replace = array
(
'account_status'        => $data['Status'],
'days_remaining'        => number_format($data['PremiumDays'])
);

$this->content = Template::Load('account-template', $replace);
}

PremiumDays 列包含如下数字 10 , 15 , 30

PremiumStartTime 包含此格式的日期 2018-12-17 21:13:00

我要做的是显示保险费的实际剩余天数 days_remaining 预产期 根据第二列,从保费开始算起的天数 起始时间前

像这样的:

days_remaining = PremiumDays - (NumberOfDaysSincePremiumStarted(DateToday - PremiumStartTime))