how to calculate date by adding days?
['As a cell reference, e.g. =A2 + 10.', 'Using the DATE(year, month, day) function, e.g. =DATE(2015, 5, 6) + 10.', 'As a result of another function. For example, to add a given number of days to the current date, use the TODAY() function: =TODAY()+10.']