if you actively began your business - example took action to get customers. you can write off post-opening expenses. pre-opening expenses are subject to IRC section 195

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

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.

example start-up costs incurred of $54,000 then $1000 deductible 1st year the balance amortized over 180 months 

“start-up expenditure” means any amount—
(A)paid or incurred in connection with—
(i)investigating the creation or acquisition of an active trade or business, or
(ii)creating an active trade or business, or
(iii)any activity engaged in for profit and for the production of income before the day on which the active trade or business begins, in anticipation of such activity becoming an active trade or business, and
(B)which, if paid or incurred in connection with the operation of an existing active trade or business (in the same field as the trade or business referred to in subparagraph (A)), would be allowable as a deduction for the taxable year in which paid or incurred.