I am using Javascript to derive the first and last day of the month based on a starting date.  Then, using the last day of the month; add that number of rows to a table.  The table fields include date and day of the week for each day in the month.  I will attach the code that is working for me so far.  The problem, is that the last day of the week in the table field; throws a calculation error. 

I need some ideas on what it is doing that I can change to fix the error.  Everything else is working great.

thanks for the help