Timestamps and strftime

Hi, I am having a problem trying to use a date that I take from a MySQL database. The field is defined as a timestamp in the database. I extract it using PDO and I am trying to use the value with strftime as follows: foreach ($stmt->fetchall(PDO::FETCH_ASSOC) as $row) { echo strftime("%j",$row['UpdateDate']); } but it returns the following error: PHP Notice: A non well formed numeric value encountered... I understand that this is because strftime is expecting a numeric value and that ,$row['UpdateDate'] is a character but I am not sure how to resolve the issue. Any help would be apprecaited. TIA, JC