How to Fix Excel 365 mmm Date Format Showing Sept Instead of Sep
Question details
Users need to format dates to display the standard three-letter month abbreviation ('Sep') instead of the four-letter abbreviation ('Sept') that currently appears when using the 'mmm' format code.
- Product
- Microsoft Excel 365
- Device & OS
- not provided
- Scenario
- Applying a custom 'mmm' date format to spreadsheet cells in order to display a shortened month name.
- Observed behavior
- The 'mmm' custom format incorrectly outputs a four-letter abbreviation ('Sept' instead of 'Sep') due to recent Unicode locale modifications.
Identify the cells or formula outputs that require the specific three-letter month abbreviation, as you will need to apply a new custom locale tag to correct the display.
Apply Locale-Qualified Custom Format Codes
Use a region-specific locale tag within the custom formatting menu to force Excel to display standard three-letter month abbreviations.
Excel 365 relies on system and Unicode locale settings to determine date abbreviations. A recent update to these Unicode standards caused the 'mmm' format for September to default to four letters ('Sept') in some configurations. By manually qualifying the locale inside your custom format code, you can override this behavior and restore the traditional 'Sep' display.
Highlight the cells containing the dates that are currently displaying 'Sept'.
Right-click the selected cells and choose 'Format Cells' from the context menu, or simply press the Ctrl + 1 keyboard shortcut.
Navigate to the 'Number' tab and select 'Custom' from the category list on the left. In the 'Type' text box, enter the format [$-409]dd-mmm-yyyy (this specifies US English).
Click 'OK' to save your changes. The dates will now force the three-letter 'Sep' abbreviation.
Use WPS Spreadsheet for Reliable Formatting
Avoid unexpected formatting changes caused by sudden software updates. WPS Office is a free, lightweight alternative to Microsoft Office that provides predictable custom formatting rules and full compatibility with your existing workbooks.
- 1. Download WPS Office: Visit the official WPS Office website and download the free installer for your operating system.
- 2. Install the application: Run the setup file and follow the on-screen instructions to install the software.
- 3. Open your spreadsheet: Launch WPS Spreadsheet and open your existing Excel file to enjoy stable and predictable date formatting.
Frequently Asked Questions
Why did Excel 365 change the 'mmm' format to output four letters?
This display change occurred due to recent updates in the underlying Unicode Common Locale Data Repository (CLDR) adopted by Microsoft. The standard abbreviation for September in certain language locales was updated to 'Sept', overriding the traditional three-letter display.
Will this formatting issue affect my spreadsheet formulas?
Yes, if you use functions like TEXT(A1, "mmm") to extract the month name and run logical checks (like IF statements) against the specific text 'Sep', those formulas may fail or return errors when Excel outputs 'Sept' instead.
Can I permanently change the default date format back to three letters?
You cannot change the underlying Unicode behavior natively, but creating a custom cell style with the locale-qualified code (e.g., [$-409]mmm) and applying it to your workbook will consistently enforce the three-letter rule.
