In Excel, dates are serial numbers. Generally, the best way to enter valid dates is to use cell references, as shown in the example. If you want to enter valid dates directly inside a function, the DATE function is the best approach.
The basis argument controls how days are counted. The PRICE function allows 5 options (0-4) and defaults to zero, which specifies US 30/360 basis. This article on Wikipedia provides a detailed explanation of available conventions.