The Excel Eomonth function returns the last day of the month, that is a specified number of months before or after an initial supplied start date.
The syntax of the function is:
where the arguments are as follows:
start_date    The initial date 
months    The number of months to add to (or subtract from) the start_date, before returning the last day of the resulting month 
Note that, when supplying a date to a function, this should be input as either:
Warning:
  If you attempt to input a date argument as text, the interpretation of this may differ, depending on the date system and date interpretation settings on your computer. 
  Although you can enter dates as serial numbers, this is not recommended, as date serial numbering varies across different computer systems. 
The following spreadsheet shows examples of the Eomonth function.
The formulas are shown in the spreadsheet on the left and the results are shown in the spreadsheet on the right.
Formulas:

Results:

Note how you can use negative values for the months argument, to get a date that is before the supplied start_date. You can also set the months argument to zero, to get the last day of the original start_date.
Further examples of the Excel Eomonth function can be found on the Microsoft Office website.
If you get an error from the Excel Eomonth function this is likely to be one of the following :
#VALUE!    Occurs if either any of the supplied arguments are not numeric values. 
#NUM!   
Occurs if either:
