WEEKNUM
WEEKNUM は、内部日付番号が、1 年の何週目であるかを返します。
国際標準 ISO 8601 は、月曜を週の最初の日とすべきであると宣言しています。2 年にまたがる週には、日数が多いほうの年で番号が割り当てられます。つまり、どの年の 1 週目も、1 月 4 日を含みます。
This function is part of the Open Document Format for Office Applications (OpenDocument) standard Version 1.2. (ISO/IEC 26300:2-2015)
ISOWEEKNUM(Number)
数値 には日付のシリアル値を入力します。
When entering dates as part of formulas, slashes or dashes used as date separators are interpreted as arithmetic operators. Therefore, dates entered in this format are not recognized as dates and result in erroneous calculations. To keep dates from being interpreted as parts of formulas use the DATE function, for example, DATE(1954;7;20), or place the date in quotation marks and use the ISO 8601 notation, for example, "1954-07-20". Avoid using locale dependent date formats such as "07/20/54", the calculation may produce errors if the document is loaded under different locale settings.
Unambiguous conversion is possible for ISO 8601 dates and times in their extended formats with separators. If a #VALUE! error occurs, then unselect Generate #VALUE! error in LibreOffice - PreferencesTools - Options - LibreOffice Calc - Formula, button Details... in section "Detailed Calculation Settings", Conversion from text to number list box.
=ISOWEEKNUM(DATE(1995;1;1)) returns 52. Week 1 starts on Monday, 1995-01-02.
=ISOWEEKNUM(DATE(1999;1;1)) returns 53. Week 1 starts on Monday, 1999-01-04.