To switch to a custom function I need to get the subscription key of a particular subscription. I referenced the documentation where it shows how to get the key, but I failed to include it in my code. So what my code does is I hook in processed subscription payment with my function when a renewal is triggered. So this code only runs when you pay for a subscription renewal.
add_action( 'processed_subscription_payment', 'callupdater' );
//need to get the subscription key here to pass to updatedays()
$key = .....
//do some tasks with the key