We've introduced how to automatically display a "New Arrival" icon on a Shopify collection page for a set period of time after a product is registered, but this is how to set it up if you want to have the same functionality on your product details page in conjunction with that.
The date the product was published will be the base date. (If a published product is made private and then republished, the date will become the publication date, so it will be treated as a new product and will be eligible for the "New product icon display.")