The EOMONTH function is a date and time function which returns the last day of the month the specified number of months before or after the input date. Setting the number of months to zero allows you to return the last day of the input month.

This is the format of the EOMONTH function:

=EOMONTH(INPUT DATE, NUMBER OF MONTHS)

Here is an example of the EOMONTH function:

=EOMONTH(45054,1) = 45107 = June 30, 2023 

This function is valuable when determining a variety of validity periods, budget dates, and tracking metrics. Especially when considering that business periods will refer to closing and opening dates as being one day apart.

An example of a functional implementation of this would be taking note of all transactions which occur during a month for budgeting and expense purposes. You are using a SUMIFS to return all transactions during each period with begins at the start of each month, and ends on the last day of each month.

ex: The period start dates are in column C, and the data starts on row 3

=EOMONTH(C3,0)