Date Help

EXISTING CODE FOR BANANAS:
if ($player_id == $account_id) {
    $bananaspurchase_more = "<br>Purchased on: $bananas_purchased";
} else {
    $bananaspurchase_more = "";
}

HTML FOR HOW IT SHOWS UP ON A PLAYER'S HOMEPAGE:
<tr valign=top><td><b>Bananas:</td><td>$bananas available [$$bananas_fee each: $bpublic] $bananaspurchase_more</td></b>

WHAT I ATTEMPTED:
$date=date_create("$bananas_purchased");
date_sub($date,date_interval_create_from_date_string("30 days"));
echo date_format($date,"Y-m-d");

ALSO ATTEMPTED TO GET IT TO ADD 30 DAYS TO THE EXISTING DATE IN THE SQL TABLE FOR BANANAS:
$date=date_create("$bananas_purchased");
date_add($date,date_interval_create_from_date_string("30 days"));
echo date_format($date,"Y-m-d");