WEEKNUM_ADD

Returns the US convention week number of a given date.
The US convention defines the week containing January 1st to be the first week of the year.

link - help.libreoffice.org/latest/en-US/text/scalc/01/func_weeknum.html 

WEEKNUM_EXCEL2003
WEEKNUM_OOO

Excel - ISOWEEKNUM

Returns the European convention week number of a given date.
The European convention however defines the first week as the first one containing four or more days.