In zen-cart what determines if a product is "New" (New Products for XXXXX Month)

There are settings in in Admin->Configuration->Maximum Values to determine "what" New Product means ...

New Product Listing - Limited to ...
Limit the New Product Listing to
0= All Products
1= Current Month
7= 7 Days
14= 14 Days
30= 30 Days
60= 60 Days
90= 90 Days
120= 120 Days

ALTERNATIVE:

If, however, you wish to hand-pick which items show up, then turn OFF the "New Products" box, and use the "Featured Products" instead.  See Admin->Catalog->Featured to pick the products.