Load the invoice by
$invoice = Mage::getModel('sales/order_invoice')->loadByIncrementId($invoiceIncrementId);
and then get the invoice date using
$createdDate = $invoice->getCreatedAt();
and put $createdDate and modify the below code from
$page->drawText(
Mage::helper('sales')->__('Order Date: ') . Mage::helper('core')->formatDate(
$order->getCreatedAtStoreDate(), 'medium', false
),
35,
($top -= 15),
'UTF-8'
);
To
$page->drawText(
Mage::helper('sales')->__('Invoice Creation Date: ') . Mage::helper('core')->formatDate(
$createdDate, 'medium', false
),
35,
($top -= 15),
'UTF-8'
);
NOTE :-
If you have multiple invoices for the same order then you can get the all invoice increment id by
$_invoices = $_order->getInvoiceCollection();
foreach($_invoices as $_invoice){
$_invoice->getIncrementId() = $_invoice->getIncrementId();
}