There are formulas in Excel to take a date, and return the day of the month, the month of the year, or the year itself. Of course, you can also format a date, so it stays as a date, but only displays the day, month, or year, but sometimes you need just the number and not the whole date, and that is where DAY, MONTH, and YEAR come in.
=DAY(serial_number) The input for the DAY function is technically a serial_number. This is the way that Excel stores dates, where 1 Jan 1900 is 1, 2 Jan 1900 is 2, etc. Basically, if you have a date in a cell you can use that as input to the DAY function, but just be careful that you actually have dates and not things that look like dates, but are actually text. A good way to make sure you have a date, if you are entering today’s date, use the shortcut key Ctrl + ;
The output for DAY will be a number between 1 and 31, i.e. the day of the month. If you want the number of days since the start of the year, you can do that with a combination of YEAR and another function called DATE. There is an excellent explanation of how to do that here.
As you can see the input for MONTH, and YEAR are the same as for DAY, and the output is pretty obvious.
Once you have your numbers, you can use them in lookup formulas, to do calculations, or whatever else you need.