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## 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.

### Syntax

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)

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.

### Example

=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.