Will Amazon deliver order within same day if I set handling time to 0 day?

Handling Time matters when you are using Merchant fulfillment or FBM account because in FBM listing when you set the Handling Time to 0 that means you have to dispatch the orders within the same day but in FBA if you have the Prime Delivery option to your customers only then Amazon will deliver the order within a day or Same day.

yeah only if you’re having a prime delivery subscription

It’s not guaranteed that Amazon will deliver an order the same day even if the handling time is set to 0. Setting the handling time to 0 means that the seller is ready to ship the product immediately upon receiving an order, but the actual delivery time will depend on a number of factors, including the shipping method selected by the customer and the delivery address.