The best way to achieve this is to implement your own InsertDateTimeDialog and mark it with the [CustomInsertDateTimeDialog] attribute. As RadRichTextBox loads all dialogs through MEF, this will ensure that the custom implementation will be used instead of the default one.
Please find attached a zip with a possible implementation of a CustomInsertDateTimeDialog. I have just taken the default implementation and changed the name of the class and attribute from "Default~" to "Custom~". All you need to do is to change patterns added in the GetDateFormatPatterns method.
I hope this helps.
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