Most small businesses use tax-accounting for financial reporting purposes. However, some businesses may choose to use Generally Accepted Accounting Principles (GAAP) or even International Financial Reporting Standards (IFRS). Any business using these alternative financial reporting standards must adopt new rules (ASC 606 / IFRS 15) for 2018 (reporting periods starting December 15, 2018 for GAAP; January 1, 2019, for IFRS). Here’s a good article explaining the impact on small businesses.