/text/scalc/01/func_weeknum.xhp WEEKNUM function WEEKNUM WEEKNUM calculates the week number of the year for the internal date value as defined in ODF OpenFormula and compatible with other spreadsheet applications. Supported are two week numbering systems: System Description System 1 The week containing January 1 is the first week of the year, and is numbered week 1. System 2 The week containing the first Thursday of the year is the first week of the year, and is numbered week 1. That means that week number 1 of any year is the week that contains January 4th. ISO 8601 defines this system and that the week starts on Monday. WEEKNUM(Number [; Mode]) Number is the internal date number. Mode sets the start of the week and the week numbering system. This parameter is optional, if omitted the default value is 1. System Mode Day of the week System 1 1 Sunday 2 Monday 11 Monday 12 Tuesday 13 Wednesday 14 Thursday 15 Friday 16 Saturday 17 Sunday System 2 21 Monday (ISO 8601) 150 (ISO 8601, for interoperability with Gnumeric) =WEEKNUM(DATE(1995;1;1);1) returns 1 =WEEKNUM(DATE(1995;1;1);2) returns 52. If the week starts on Monday, Sunday belongs to the last week of the previous year. =WEEKNUM(DATE(1995;1;1);21) returns 52. Week 1 starts on Monday, 1995-01-02. =WEEKNUM(DATE(1999;1;1);21) returns 53. Week 1 starts on Monday, 1999-01-04.