The WEEKNUM function is a date and time function which returns the calendar week number of the input date based on the day of the week which is considered the ‘start’ of each week. That start day can be defined in the function.

This is the format of the WEEKNUM function:

=WEEKNUM(DATE VALUE, RETURN TYPE)

Here is an example of the WEEKNUM function:

=WEEKNUM(43335,1) = 34 

This function is valuable when tracking over time metrics. While analyzing day to day values may be required under certain circumstances, grouping data by week, month, quarter, or year can lead to more informative results. The WEEKNUM function allows for this style of consolidated analysis.

An example of a functional implementation of this would be tracking sales over a two-year historical period to help determine budget requirements for the following year. Using the YEAR function as well as the WEEKNUM function will allow you to overlay both years information at similar time periods, giving you a true picture of where you are trending for the following twelve months.

=WEEKNUM(43854,1) = 4