Using the Excel DAY Function
SummaryThe DAY function allows you to pass the function a date as a serial number and it will return the day of the month as an integer between 1 and 31. DAY can also be used to provide a value to another function, such as DATE.
A required value that is the date of the day you are trying to find. If dates are entered as text you may encounter errors. To avoid this, enter dates using the DATE function or the output of other functions and formulas that are in serial number format.
Excel stores dates in serial number format so that they can be easily used in calculations. Serial number 1 is representative of January 1, 1900 and August 15, 2018 is 43327 since it is 43,327 days after January 1, 1900.