We have checked your file in MS Excel, MS Excel shows the custom pattern as “m/d/yy;” too, please see the attached screenshot. And in your template file, if you check the string saved in the xml file, it is “m/d/yy;” too. So we don’t think it is an issue for the returned custom string. And for the formatting code(pattern) of Style.Custom, the syntax is same with MS Excel, not any program with language.
If you need one new feature that support converting the format pattern of MS Excel to specific language, such as Java, we are afraid it is complicated and we cannot support it soon. As a component, we have to take all possible situations into consideration when implementing it. You may consider to convert some special patterns by themselves according to your requirement.
Anyways, if you still think this is a valuable feature and hard to solve it by yourselves, we can create a new feature request ticket and try to support it later on.
custom.png (42.4 KB)