Explanation
The 2nd argument (months) of the EOMONTH function allows you to get the last day of the month in future or past months. When you use zero for months, EOMONTH will return the last day of the month in the same month.
To get the last day of the prior month, use:
=EOMONTH(date,-1)
To get the last day of the next month, use:
=EOMONTH(date,1)
Alternative formula
You can also write a formula using the DATE, YEAR and MONTH functions to return the last day of the month:
=DATE(YEAR(date),MONTH(date)+1,0)
The trick with this formula is supplying zero for the day. When you supply zero as the day argument to DATE, the date function will "roll back" one day to the last day of the previous month. So, by adding 1 to the month, and using zero for day, DATE returns the last day of the "original" month.
Although EOMONTH is a more convenient function, it won't accept a range of dates in an array formula. In that case, you can use the alternative above.
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EOMONTH Function
The Excel EOMONTH function returns the last day of the month, n months in the past or future. You can use EOMONTH to calculate expiration dates, due dates, and other dates that need to land on the last day of a month. Use a positive value for months to move forward in time, and a negative number...
DATE Function
The Excel DATE function creates a valid date from individual year, month, and day components. The DATE function is useful for assembling dates that need to change dynamically based on other values in a worksheet.
