>>(1) Your formula is not very good. If the day is 31, if won't work
>
>The current value is 01/08/2015. I changed that for 31/08/2015. I did not have any problem.
>

OK., should have said that it won't always work


Try to add 6 months to 31/08/2015 - that would become 31/02/2016