the taxpayer shall be allowed a deduction for the taxable year in which the active trade or business begins in an amount equal to the lesser of—
(i)the amount of start-up expenditures with respect to the active trade or business, or
(ii)$5,000, reduced (but not below zero) by the amount by which such start-up expenditures exceed $50,000, and
(B)the remainder of such start-up expenditures shall be allowed as a deduction ratably over the 180-month period beginning with the month in which the active trade or business begins.
the IRS does not seem to have rules for determining when a business begins but this is from IRS reg 1.195-1 which deals with start-up costs
for the taxable year in which the active trade or business to which the expenditures relate begins.
to me since the use of "active" in the phrase that means when the doors to the business open - ie you have done something to notify potential customers that you're ready to sell to them
however, an article by a CPA firm says in the case of an LLC when it files with the state though they do hedge a bit by saying it could be on a date the IRS determines
https://www.cookcpagroup.com/business-start-for-tax-purposes/
It is important to note that the start date of a business can change depending on other factors. For example, under certain circumstances, the IRS may analyze a company’s activities to determine whether they are liable for taxes.
IRS PUB 535 contains no examples for a new business.
