To retrieve the month and day of the oldest post on your blog via PHP (which you could of course include in a function) is relatively simple – in essence all that you are doing is putting in a call for the WordPress engine to return to you the oldest post in the database and then extract the relevant information from there.

The code is as follows:

//Default values for the resulting variables
$oldestyear = date('Y');
$oldestmonth = date('m');
//Calculate Oldest Post Date
$args = array('orderby'=>'date','order'=>'ASC','posts_per_page'=>1,'caller_get_posts'=>1);
$oldestpost = get_posts($args);

//Extract the date information            
if (!empty($oldestpost)){
    $oldestyear = mysql2date('Y',$oldestpost[0]->post_date);
    $oldestmonth = mysql2date('m',$oldestpost[0]->post_date);

//Print out the result            
echo "

The oldest post of this blog appeared in the $oldestmonth month of $oldestyear


As simple as that.