Once you purchase a item the seller cannot change the estimated delivery date so you always can file an item not received claim for up to 30 days after the delivery date that was on the listing the day you made the purchase. That delivery date should show up on your purchase history and shouldn’t change. If you are looking at the listing itself (rather than your purchase history) that date will change each day as that delivery date is based on if you purchased the item today.
if you paid with PayPal you have 180 days from the date of purchase to file a claim. If you paid with a credit card you can contact them to start a chargeback.