Choosing a taxable year (fiscal year) for U.S. corporations is crucial, especially for pass-through entities like LLCs and S-Corps. Unlike Japanese companies which often use an April-to-March fiscal year, U.S. pass-through entities predominantly adopt a January-to-December calendar year to align with individual tax reporting. This article thoroughly explains the underlying U.S. tax laws, options and restrictions for each entity type, detailed case studies, advantages and disadvantages, and common pitfalls. With explanations of technical terms and practical advice, it aims to provide a complete understanding of this complex topic.