Excel uses two different date systems to handle calendar dates. These are the 1900 date system and the 1904 date system. The main difference is their starting point. The 1900 system begins on January 1, 1900. The 1904 system starts on January 1, 1904. This creates a gap of 1,462 days between the two systems.

The Excel 1900 date system is the default for Windows. It counts January 1, 1900 as day 1. This system has a known error. It incorrectly treats the year 1900 as a leap year. This was done for compatibility with older programs like Lotus 1-2-3. So, February 29, 1900, exists in this system even though it is not a real date.

The Excel 1904 date system is the traditional default for Mac. It counts January 1, 1904 as day 0. This system avoids the leap year error. It does not include February 29, 1900. This makes it more accurate for historical dates before March 1, 1900.

The difference between Excel 1900 and 1904 date systems affects your work. If you open a file made with one system in the other, dates can appear wrong. They may shift by four years and one day. This is a big problem for Excel date system compatibility. It can cause errors in budgets, schedules, and reports.

You can check which system your workbook uses. Go to File > Options > Advanced. Look for "When calculating this workbook". You will see "Use 1904 date system". If it is checked, you are using the 1904 system. If not, you are using the 1900 system.

Changing the date system in Excel is simple but has risks. To change it, go to the same options menu. Check or uncheck the "Use 1904 date system" box. But be careful. When you change the system, all your dates will change. They will move by 1,462 days. This can break your formulas and charts.

The Excel 1900 date system has issues with early dates. Any date before March 1, 1900, is not valid. This is because of the leap year mistake. The Excel 1904 date system does not have this problem. It can correctly handle dates from January 1, 1904, onward. This is one of the Excel 1904 date system benefits.

You might need Excel date system conversion. For example, if you get a file from a Mac user and you use Windows. To fix the dates, you can add or subtract 1,462 days. Use a formula like =A1+1462 to convert from 1904 to 1900. Use =A1-1462 to convert from 1900 to 1904. Always make a backup before you do this.

The Excel date system for Mac changed in recent years. New versions of Excel for Mac now often use the 1900 system by default. This helps with compatibility. But older files may still use the 1904 system. The Excel date system for Windows still defaults to 1900.

The Excel date system default can be confusing. It depends on your Excel version and your computer. Always check the setting when you start a new project. This is especially important if you share files with others.

Knowing the Excel date system history helps understand why we have two systems. The 1900 system came from Lotus 1-2-3. The 1904 system was made for early Mac computers. Microsoft kept both for compatibility with old files.

When you work with dates in Excel, always be aware of the date system. It affects functions like TODAY() and NOW(). It also changes how Excel works with other programs. For example, if you export dates to a database, they might be wrong if the systems do not match.

To avoid problems, talk with your team. Agree on which date system to use. This prevents Excel date system compatibility errors. It saves time and prevents mistakes in your data.

In summary, Excel date systems are important. They change how Excel stores and calculates dates. The 1900 system has a leap year error. The 1904 system does not. The difference is 1,462 days. Always check your workbook setting to avoid errors.