The IRS has a sort-of definitive answer about when a business begins, and it can vary.

Ordinarily, a corporation begins business when it starts the business operations for which it was organized;... on the date of its incorporation. Mere organizational activities, such as the obtaining of the corporate charter, are not alone sufficient to show the beginning of business.

However, the IRS also states that the process of beginning the business such as buying necessary operating assets, inventory, 'setting up shop' can also be a sign that a business has begun. To learn more on Starting a Business from the IRS please see Starting A Business.  So you can use the start date of the date you incorporate or when you began buying inventory.