Question

Does anyone know the formula to add dates on a calender together on Excel?

Answer 1

You can add some number of days to a date by simply using the =SUM function. Since Excel stores dates as
a number of days, no further work is required. For example, to add 5 days to 1/1/98, in A1, use =A1+5,
which gives 1/6/98.

To add a number of months or years to a date, you first need to decompose the initial date into its year, month,
and day components, add in the desired offset, and then have Excel put the components back together.
For example, say you have a date in A1, to which you want to add 3 months and 4 days. You'd use the following formula:

=DATE(YEAR(A1), MONTH(A1)+3, DAY(A1)+4)