Week numbers in LibreOffice Calc
How to get the week number from a date
To get the ISO week number (1-53) for a date in cell A1, use
21 is a code that identifies ISO week numbers among other week numbering scheme.
To get the corresponding year, use
=YEAR(A1 - WEEKDAY(A1; 2) + 4).
How to get the date from a week number
To get the date of the Monday in a week, use
=DATE(A1; 1; -3 + 7 * B1 - WEEKDAY(DATE(A1; 1; 4); 2) + 1).
Cell A1 contains the four-digit year (e.g. 2013), and cell B2 contains the week number (1-53).
How to get the number of weeks in a year
To get the of ISO weeks in a year (i.e. the number of the last week), use
Cell A1 contains the four-digit year (e.g. 2012).