Article 51 allows taxpayers to change their accounting year-end date, subject to the prior consent of the Chairman of the Tax Authority. When a change occurs, it creates a 'transition period'—the duration between two non-equal consecutive periods. The income earned during this transition period is treated as the taxable income for the tax year in which the period ends. This mechanism ensures that the Tax Authority can monitor and approve changes to reporting cycles, preventing the strategic shifting of income to avoid tax through period manipulation.