If your buying a NEW product SKU. Your never asked to put in a key so we would not be able to determine time left.
If you are purchasing a Renewal SKU and upgrade to a different renewal SKU, you will not get to bring over your time left.
If your are purchasing a renewal for the same product SKU and input the key when asked, time left is brought over if the key has 30 days or less left on it.
So don't renew until you have 30 days or less.
Its just better to wait for the client to pop up a message. Because that automates the purchase process and moving the time over.
Going to the website and manually renewing will not guarantee the time gets moved over.
So just wait for the client to tell you its time to renew and click on the purchase button from that message to ensure time is moved over the product.