>>Use the formula edate(startdate, numberOfMonths)
>>
>>looks like
>>

>>=EDATE(A2,4)
>>

>
>Yes, on column 2, I have something like this:
>
>=DATE(YEAR(A1),MONTH(A1)+8,DAY(A1))
>
>So, column 2 is 8 months older than column 1.
>
>But, on column one is the value and that value is 42217. So, directly on that, how can I know what date it is and how can I add directly the number of months on that date?

(1) Your formula is not very good. If the day is 31, if won't work

(2) that value is 42217. So, directly on that, how can I know what date
Rightclick cell - format - take Date. That will make it display a date

(3) how can I add directly the number of months on that date

In the same cell ? Say 8 months

=edate(42217, 8)

In another cell - say 42217 is in cell A2

=edate(A2, 8)

You'll have to format the cell to display a date