Why don't astronomers use everyday units to measure distances (what is an AU or a pc)? (Intermediate)
How this formula works: The WEEKNUM function takes two arguments, a date, and, optionally, an argument called return_type, which controls the scheme used to calculate the week number. See the WEEKNUM page for more information. B4 contains a date value for January 5, 2016. The DAY function returns the number 2 representing the calculated week number. Note that by default, the WEEKNUM function uses a scheme where week 1 begins on January 1, and week 2 begins on the next Sunday (when the return_type argument is omitted, or supplied as 1).
If you type a number that resembles a date, for example a fraction 12/7 then Excel will probably think it is a date and make it 7-Dec. If you just format the cell afterwards, then you will get a number that is the date as a serial number. Not what you want and you will have to enter your number again. So, format your cells first for numbers if you are going to be entering any number that could resemble a date. Excel really likes to make things dates.