# Date calculations in Excel

Excel supports two date systems. Each date system uses a unique starting date from which all other workbook dates are calculated. Excel 2008 for Mac and earlier Excel for Mac versions calculate dates based on the 1904 date system. Excel for Mac 2011 uses the 1900 date system, which guarantees date compatibility with Excel for Windows. All versions of Excel for Windows calculate dates based on the 1900 date system.

However, when you copy dates from an earlier version of Excel for Mac and try to paste them into Excel for Mac 2011, you receive a message that all dates will shift by approximately four years unless you convert the data. You have two options. You can convert the dates to use the 1900 date system (recommended). This option makes the dates compatible with other dates in the workbook. Or, you can keep the 1904 date system for the pasted dates only.

 Note  Dates that are copied as part of a chart cannot be converted from the 1904 date system and will vary by approximately four years. You must convert these dates manually.

## The 1900 date system

In the 1900 date system, dates are calculated by using January 1, 1900 as a starting point. When you enter a date, it is converted into a serial number that represents the number of days elapsed since January 1, 1900. For example, if you enter July 5, 2011, Excel converts the date to the serial number 40729. This is the default date system in Excel for Mac 2011. If you choose to convert the pasted data, Excel adjusts the underlying values, and the pasted dates match the dates that you copied.

## The 1904 date system

In the 1904 date system, dates are calculated by using January 1, 1904 as a starting point. When you enter a date, it is converted into a serial number that represents the number of days elapsed since January 1, 1904. For example, if you enter July 5, 2011, Excel converts the date to the serial number 39267. This is the default date system in earlier versions of Excel for Mac. If you choose not to convert the data and keep the 1904 date system, the pasted dates vary from the dates that you copied.

## The difference between the date systems

Because the two date systems use different starting days, the same date is represented by different serial numbers in each date system. For example, July 5, 2011 can have two different serial numbers, as follows:

Date System

Serial number

1900

40729

1904

39267

The difference between the two date systems is 1,462 days. This means that the serial number of a date in the 1900 date system is always 1,462 days greater than the serial number of the same date in the 1904 date system. 1,462 days is equal to four years and one day (including one leap day).

## Set date system preferences

If you frequently work with files that were created in Excel 2008 for Mac and prefer not to change the date system or to encounter this message every time, you can set the 1904 date system as the default.

1. On the Excel menu, click Preferences.

2. Under Formulas and Lists, click Calculation  .

3. Under Workbook options, select the Use the 1904 date system check box.

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