EOMONTH Problem And Alternative

I've recently had problems with files using the EOMONTH function in 2007. Files with the function in sometimes come up with the following warning message:



Once the file opens the cell with the EOMONTH function in just displays =#N/A
As a result I've created the following function to replace the EOMONTH function. The function uses the same criteria as EOMONTH but uses the word Lastday. The function is used like =Lastday(start_date,months)

Public Function Lastday(datex As Date, mthpls As Integer)

mthpls = mthpls +1

If Month(datex) + mthpls > 12 Then
mth = Month(datex) + mthpls - 12
yr = Year(datex) + 1
Else
mth = Month(datex) + mthpls
yr = Year(datex)
End If
dy = 1
Dim dayx As String
Lastday = Format(DateSerial(yr, mth, dy) - 1, "Short Date")


End Function

It's not fully tested so if you decide to use it and find a problem let me know.