Accounting Act Article 15

The beginning and the end of a fiscal year shall be determined by the Budget Act.A quarter of a fiscal year shall be three months beginning from the date in which the fiscal year is commenced.Each month of a fiscal year shall be determined according to calendar month.Each month shall be divided into three periods, namely the period from the 1st to the 10th day, the period from the 11th to the 20th day, and the period from the 21st to the last day of each month.Each month shall be sub-divided into the period of five days each as a sub-period, the 1st to the 5th day of each month is the first sub-period, and the 26th to the last day of each month is the last sub-period.Periods that do not commence with the first day of a fiscal year or the first day of calendar month, or months that are not consecutive, shall be determined pursuant to Article 121 to 123 of the Civil Code.
中文原文 會計法第15條
會計年度之開始及終了,依預算法之所定。
會計年度之分季,自年度開始之日起,每三個月為一季。
會計年度之分月,依國曆之所定。
各月之分旬,以一日至十日為上旬,十一日至二十日為中旬,二十一日至月之末日為下旬。
各月之分為五日期間者,自一日起,每五日為一期,其最後一期為二十六日至月之末日。
期間不以會計年度或國曆月份之始日起算者,或月份非連續計算者,其計算依民法第一百二十一條至第一百二十三條之所定。

English translation for reference only; the Chinese text prevails. Not legal advice.