Software & Apps MS Office Serial Number and Serial Date in Excel How dates get calculated in Excel worksheets By Ted French Writer Former Lifewire writer Ted French is a Microsoft Certified Professional who teaches and writes about spreadsheets and spreadsheet programs. our editorial process Ted French Updated July 08, 2019 Pongsak Tawansaeng / Getty Images MS Office Excel Word Powerpoint Outlook Tweet Share Email The serial number, or serial date, is the number Excel uses to calculate dates and times entered into a worksheet. The serial number is calculated either manually or as a result of formulas involving date calculations. Excel reads the computer's system clock to keep track of the amount of time that has elapsed since the date system's start date. The information in this tutorial applies to Excel 2019, Excel 2016, Excel 2013, Excel 2010, Excel 2007, and Excel for Mac. Two Possible Date Systems By default, versions of Excel that run on Windows store the date as a value representing the number of full days since midnight January 1, 1900, plus the number of hours, minutes, and seconds for the current day. Versions of Excel that run on Macintosh computers default to one of the following two date systems: Excel for Mac versions 2019, 2016, and 2011: The default date system is the 1900 date system which guarantees date compatibility with Excel for Windows.Excel 2008 and older versions: The default date system begins on January 1, 1904 and is referred to as the 1904 date system. All versions of Excel support both date systems and it's possible to change from one system to the other. Serial Number Examples Mohd Azri Suratmin / EyeEm / Getty Images In the 1900 system, the serial number 1 represents January 1, 1900, 12:00:00 a.m. while the number 0 represents the fictitious date January 0, 1900. In the 1904 system, the serial number 1 represents January 2, 1904, while the number 0 represents January 1, 1904, 12:00:00 a.m. Times Stored as Decimals Times in both systems are stored as decimal numbers between 0.0 and 0.99999, where: 0.0 is 00:00:00 (hours:minutes:seconds)0.5 is 12:00:00 (12 p.m.)0.99999 is 23:59:59 To show dates and times in the same cell in a worksheet, combine the integer and decimal portions of a number. For example, in the 1900 system, 12 p.m. on January 1, 2016, is serial number 42370.5 because it is 42370 and one-half days (times are stored as fractions of a full day) after January 1, 1900. Similarly, in the 1904 system, the number 40908.5 represents 12 p.m. on January 1, 2016. 3dts / Getty Images Serial Number Uses Many projects that use Excel for data storage and calculations use dates and times in some way. For example: A long-term project that counts the number of days between current and past dates using the NETWORKDAYS function.A loan calculation that determines a future date using the EDATE function.Timesheets that calculate the elapsed time between start and end times, as well as hours, and overtime as necessary using formulas that add or subtract dates and times.Time stamping a worksheet with the current date and time with keyboard shortcuts that read the current serial number.Updating the displayed date and time whenever a worksheet is opened or recalculated with the NOW and TODAY functions. Only one date system can be used per workbook. If the date system for a workbook that contains dates is changed, those dates shift by four years and one day due to the time difference between the two date systems. Change the Default Date System To set the date system for a workbook in Excel running on a Windows PC: Open the workbook to be changed. Select File. Except in Excel 2007, where you select the Office button. Select Options to open the Excel Options dialog box. Select Advanced in the left-hand panel of the dialog box. In the When calculating this workbook section, select or clear the Use 1904 date system checkbox. Select OK to close the dialog box and return to the workbook. To set the date system for a workbook in Excel for Mac: Open the workbook to be changed. Select the Excel menu. Select Preferences to open the Excel Preferences dialog box. In the Formulas and List section, select Calculation. In the When Calculating Workbooks section, select or clear the Use 1904 date system checkbox. Close the Excel Preferences dialog box. Why Two Date Systems? PC versions of Excel (Windows and DOS operating systems) initially used the 1900 date system for the sake of compatibility with Lotus 1-2-3, the most popular spreadsheet program at the time. The problem is that when Lotus 1-2-3 was created, the year 1900 was programmed in as a leap year when in fact it was not. As a result, additional programming steps were needed to correct the error. Current versions of Excel keep the 1900 date system for the sake of compatibility with worksheets created in previous versions of the program. Utamaru Kido / Getty Images Since there was no Macintosh version of Lotus 1-2-3, initial versions of Excel for Macintosh did not need to be concerned with compatibility issues. The 1904 date system was chosen to avoid the programming problems related to the 1900 non-leap-year issue. On the other hand, it did create a compatibility issue between worksheets created in Excel for Windows and Excel for the Mac. This is why all new versions of Excel use the 1900 date system.