Hello,
To add subscription start and end date on invoice thanks page, you need to modify the file at below mentioned path.
Root/components/com_payplans/templates/default/_partials/default_partials_invoice.php, near line no. 182, find below mentioned line no of code.
<td class='span8'>
<div><?php echo XiText::_('COM_PAYPLANS_INVOICE_PRICE'); ?></div>
</td>
Replace with
<td class='span8'>
<?php $orderId = $invoice->getObjectId();
$order = PayplansOrder::getInstance($orderId);
$subscription = $order->getSubscription();
$subDate = PayplansHelperFormat::date($subscription->getSubscriptionDate(), '%m-%d-%Y');
$expDate = PayplansHelperFormat::date($subscription->getExpirationDate(), '%m-%d-%Y');
?>
<div><?php echo XiText::_('COM_PAYPLANS_INVOICE_PRICE'); ?><?php echo " ".$subDate." - ".$expDate;?></div>
</td>
Do the changes and check this. Let me know if you have any issue.