As an Amazon buyer, you can check the number of days an item has been on Amazon in the following ways:

1. View on the product detail page: Open the detail page of the item you are interested in, and the product's shelf date or sales start date will usually be displayed next to the product description and price. Although the specific form may vary depending on different regions and languages, the shelf date is usually displayed as "Date First Available" or similar terms.

2. Use Amazon filtering tools: Amazon's search results page usually provides a series of filtering tools to help you further narrow your search. On the search results page, you can use time filters, such as "Shelf Time" or "New Arrivals" to sort by the number of days the item has been on shelf. This way, you can see the most recently listed items.

3. Use Amazon product comparison tools: Amazon provides some product comparison tools that can help you compare multiple similar products. In the comparison results, you can see detailed information about each product, including the shelf date, so that you can better understand the relative newness of the product.

It should be noted that Amazon does not necessarily publicly display the exact number of days a product has been on the shelves to buyers. Sometimes, you can only know a rough time range or a relative time indication. In addition, some sellers may change the listing date or close and restart the products that were listed a long time ago, which may affect the accurate viewing of the number of days the product has been on the shelves.

However, through the above methods, you can obtain certain reference information, so that you can better understand the listing status and history of the product when purchasing.