If you initially get the string from the date() function, then pass on formatting arguments to the date-function instead:
date('Y-m-d')
instead of converting the string once again.
EDIT: If you need to keep track of the actual timestamp, then store it as a timestamp:
// Store the timestamp in a variable. This is just an integer, unix timestamp (seconds since epoch)
$time = time();
// output ISO8601 (maybe insert to database? whatever)
echo date('Y-m-d H:i', $time);
// output your readable format
echo date('j-M-Y', $time);
Using strtotime() is convinient but unessecary parsing and storage of a timerepresentation is a stupid idea.