当日期未存储为时间戳时，从MySQL检索最小和最大日期

I am trying to get the earliest and latest dates from a table field.

These are the dates as stored.

01/30/2015 02/31/2014 10/26/2014 01/31/2015

Using these queries:

$sql = "SELECT MIN(closing_date) AS first_date FROM ft_form_8 WHERE closing_date >'' 
AND status = 'closed' AND nar_id = $nar_id  ";

$sql = "SELECT MAX(closing_date) AS last_date FROM ft_form_8 WHERE closing_date >'' 
AND status = 'closed' AND nar_id = $nar_id  ";

The first_date value when retrieved is: 01/30/2015 The last_date value when retrieved is: 10/26/2014

Any suggestions?