Thanks for considering Aspose.Cells.
When custom formatting is [$-411]MMM, this formatting pattern denotes, the datetime should be formatted with ja-JP. And MS-Excel gives 1 for month January. We don’t know what’s the rule that MS-Excel uses to determine whether the month should be formatted as 1 or Jan. For Aspose.Cells component, it does not need MS-Excel and it is impossible for this component to know whether the language of MS-Excel used by one user is same with the machine’s default setting. We can only detect and use three regional settings: the default region of the application environment, the specified region set by user for one Workbook object, the specified region in the custom pattern. For the priority, region custom pattern is highest, region specified on Workbook is middle, default region of environment is lowest.