You can create a helper function to parse your custom date time string like this:
<?php
// Helper Function
function parseDate($datetime_str)
{
list($myDate, $myTime) = explode(' ', $datetime_str);
list($year, $month, $date) = str_split($myDate, 2);
return "$date/$month/$year $myTime";
}
// Data Source
$datetime_str = '140428 11:54';
// Usage
var_dump(parseDate($datetime_str));
?>
Output: