Article 5 establishes the tax year as the standard accounting period for taxpayers engaging in business activity. However, it allows for flexibility by permitting taxpayers to adopt an alternative accounting period, such as a fiscal year that does not coincide with the calendar year, provided they obtain prior approval from the General Tax Authority. The specific conditions and procedures for applying for and maintaining a non-standard accounting period are further detailed in the Executive Regulations, ensuring administrative consistency while accommodating various business cycle requirements.