If you are using eloquent model (by looking at your code, i think you are), you dont need to convert it into array. Just use it as object. Becaus elike Thomas Kim said, by default it is a Carbon instance
So it should be
$suborder['payment_date'] = $item->created_at->format('Y-m-d')
But if it is not then, you need convert it to Carbon object as Milan Maharjan answer
$createdAt = Carbon::parse($item['created_at']);