Just make a collection named new arrivals, then make the statement to only add products with the tag new arrivals,
Then add tags to all the products that are new arrivals. Once its done, just remove the tags from the products.
Have a look at https://apps.shopify.com/newr
It's an app that automatically adds products to your new arrivals collections and remove them after X days (according to your settings).
The New Arrivals Shopify app is another option that might be helpful to set up an automated tag-based new arrivals collection. It can automatically tag and untag products based on creation or publishing date. You can set up a concrete period of time the products should be considered to be "new". It will automatically remove the tag after a specified period of time.