WEEKNUM_OOO
WEEKNUM_OOO calculates the week number of the year for the internal date value.
This function exists for interoperability with Collabora Office releases older than 5.1.0 and OpenOffice.org. It calculates week numbers for a week numbering system in that week number 1 is the week that contains the January 4th. This function does not provide interoperability with other spreadsheet applications. For new documents use the WEEKNUM or ISOWEEKNUM function instead.
WEEKNUM_OOO(Number; Mode)
Number is the internal date number.
Mode sets the start of the week and the calculation type.
1 = Sunday
2 = Monday (ISO 8601)
any other value = Monday (ISO 8601)
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 Collabora Office - PreferencesTools - Options - Collabora Office Calc - Formula, button Details... in section "Detailed Calculation Settings", Conversion from text to number list box.
=WEEKNUM_OOO(DATE(1995;1;1);1) returns 1
=WEEKNUM_OOO(DATE(1995;1;1);2) returns 52. Week 1 starts on Monday, 1995-01-02.