Just leave now in your query statement and should work, no need to bind it.
Do something like :
$stmt = $dbobj->prepare(INSERT INTO tablename (id, value, time_created) VALUES (:id, :value , NOW());
$stmt->bindValue(':id' , '234' , PDO::PARAM_INT);
$stmt->bindValue(':value' , 'Sunny' , PDO::PARAM_STR);
$stmt->execute();
I hope this helps. Point if I made any mistake.