Say that you have 25 months from 6/9/2007 to complete a project, and you need to find the project end date. You could do that by using the DATE function. The DATE function has three arguments: year, month, and day. After entering 25 in cell B2, you would type this formula in cell A4:
2007 is the year argument; 6 plus the value in cell B2 is the month argument; 9 is the day argument. Commas separate the arguments, and parentheses enclose all the arguments.
The project ends on 7/9/2009. Because you typed the cell reference (B2) instead of the value in that cell (25), Excel can automatically update the result if the value changes. For example, if the time period changes from 25 months to 23 months, you could get the revised date 5/9/2009 by changing the value in cell B2 from 25 to 23, without retyping the formula.