Suppose you want to adjust a project's schedule date by adding two weeks to see what the new completion date will be, or you want to determine how long a single activity will take to complete in a list of project tasks. You can add or subtract a number of days to or from a date by using a simple formula, or you can use worksheet functions that are designed to work specifically with dates in Excel.
Add days to or subtract days from a date
Suppose that an account balance of yours is due on February 8, 2010. You want to transfer funds to your checking account so that those funds arrive 15 calendar days before that date. In addition, you know that your account has a 30 day billing cycle, and you want to determine when you should transfer funds for your March 2010 bill so that those funds are available 15 days before that date.

In cell A1, type 2/8/10.

In cell B1, type =A115.

In cell C1, type =A1+30.

In cell D1, type =C115.
Cells A1 and C1 show the due dates (2/08/10 and 3/10/10) for the February and March account balances, and cells B1 and D1 show the dates (1/24/10 and 2/23/10) by which you should transfer your funds for those due dates.
Add months to or subtract months from a date
You can use the EDATE function to quickly add or subtract a specific number of whole months to or from a date.
The EDATE function requires two values (also referred to as arguments): the start date and the number of months that you want to add or subtract. To subtract months, enter a negative number as the second argument (for example, =EDATE("2/15/10",5). This formula results in the date 9/15/09.
You can specify the value of the start date either by referring to a cell that contains a date value or by entering a date enclosed in quotation marks, such as "2/15/10".
For example, suppose you want to add 16 months to October 16, 2009.

In cell A5, type 10/16/09.

In cell B5, type =EDATE(A5,16).
The function uses the value in cell A5 as the date.

In cell C5, type =EDATE("10/16/09",16).
In this case, the function uses a date value that you enter directly, "10/16/09."
Cells B5 and C5 should both show the date 2/16/11.
Note: Depending on the format of the cells that contain the formulas that you entered, Excel might display the results as serial numbers; in this case, 2/16/11 might be displayed as 40590.
Excel stores dates as sequential serial numbers so that they can be used in calculations. By default, January 1, 1900 is serial number 1, and January 1, 2010 is serial number 40179 because it is 40,178 days after January 1, 1900.

If your results appear as serial numbers, select cells B5 and C5 and continue with the following steps.

On the Home tab, in the Cells group, click Format, and then click Format Cells.

In the Format Cells dialog box, click the Number tab.

Under Category, click Date, and then click OK. The value in each of the cells should appear as a date instead of a serial number.
Add years to or subtract years from a date
The example may be easier to understand if you copy it to a blank worksheet.
How do I copy an example?

Select the example in this article. If you are copying the example in Excel Web App, copy and paste one cell at a time.Important Do not select the row or column headers.
Selecting an example from Help

Press CTRL+C.

Create a blank workbook or worksheet.

In the worksheet, select cell A1, and press CTRL+V. If you are working in Excel Web App, repeat copying and pasting for each cell in the example.

Important For the example to work properly, you must paste it into cell A1 of the worksheet.

To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.

After you copy the example to a blank worksheet, you can adapt it to suit your needs.


How the formula works
In each of the three formulas, a specified number of years from column B is added to the year value that is derived from the date in column A.
For example, in cell A6, the YEAR function is used on the date in cell A2 (6/9/2009), and returns 2009 as the year. The formula then adds 3 (the value in cell B2) to the year value, which results in 2012. In the same formula, the MONTH function returns the value 6, and the DAY function returns the value 9. The DATE function then combines these three values into a date that is three years in the future — 6/9/2012.
You can use a similar formula to add months to a date. For example, by using the previous sample data, you can add 9 months to the date 6/9/2009 by using the following formula: =DATE(YEAR(A2),MONTH(A2)+9,DAY(A2)). This formula returns the date 3/9/2010.
Add a combination of days, months, and years to a date
The example may be easier to understand if you copy it to a blank worksheet.
How do I copy an example?

Select the example in this article. If you are copying the example in Excel Web App, copy and paste one cell at a time.Important Do not select the row or column headers.
Selecting an example from Help

Press CTRL+C.

Create a blank workbook or worksheet.

In the worksheet, select cell A1, and press CTRL+V. If you are working in Excel Web App, repeat copying and pasting for each cell in the example.

Important For the example to work properly, you must paste it into cell A1 of the worksheet.

To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.

After you copy the example to a blank worksheet, you can adapt it to suit your needs.


The formulas in the example have the following arguments (an argument is a value that is supplied to a function).
Formula to add dates
start_date: A date or reference to a cell that contains a date.
add_year: The number of years to be added.
add_month: The number of months to be added.
add_day: The number of days to be added.
How the formula works
In each of the formulas, a specified number of years, months, and days are added to the date that is contained in cell A2.
For example, in cell A5 (the second formula), the YEAR function is used on the date in cell A2 (6/9/2009), and returns 2009 as the year. The formula then adds 1 to the year value, which results in 2010. The MONTH function returns the value 6, and 7 months are added to that value. Because the total of 6 months plus 7 months is 13 months, the DATE function adds 1 year to the year value, resulting in 2011. The DATE function then subtracts 12 from the month value, resulting in a value of 1 for the month. The DAY function returns the value 9, and 5 days are added to that, resulting in 14. Finally, the DATE function combines these three values (2011, 1, and 14) into a date that is one year, seven months, and 5 days in the future — 1/14/2011.
Here are some ways you could use a formula or worksheet functions that work with dates to do things like, finding the impact to a project’s schedule if you add two weeks, or time needed to complete a task.
Let’s say your account has a 30day billing cycle, and you want to have the funds in your account 15 days before the March 2013 billing date. Here’s how you would do that, using a formula or function to work with dates.

In cell A1, type 2/8/13.

In cell B1, type =A115.

In cell C1, type =A1+30.

In cell D1, type =C115.
Add months to a date
We’ll use the EDATE function and you’ll need the start date and the number of months you want to add. Here’s how to add 16 months to 10/24/13:

In cell A1, type 10/24/13.

In cell B1, type =EDATE(A1,16).

To format your results as dates, select cell B1. Click the arrow next to Number Format, > Short Date.
Subtract months from a date
We’ll use the same EDATE function to subtract months from a date.
Type a date in Cell A1 and in cell B1, type the formula =EDATE(4/15/2013,5).
Here, we’re specifying the value of the start date entering a date enclosed in quotation marks.
You can also just refer to a cell that contains a date value or by using the formula =EDATE(A1,5)for the same result.
More examples
To add years to or subtract years from a date
Start Date 
Years added or subtracted 
Formula 
Result 
10/24/2013 
3 (add 3 years) 
=DATE(YEAR(A2)+B2,MONTH(A2),DAY(A2)) 
10/24/2016 
10/24/2013 
5 (subtract 5 years) 
=DATE(YEAR(A4)+B4,MONTH(A4),DAY(A4)) 
10/24/2008 