Week numbers in Google Sheets
How to get the week number from a date
To get the ISO week number (1-53) from 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
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
=ISOWEEKNUM(DATE(A1, 12, 28)).
Cell A1 contains the four-digit year (e.g. 2012).