Currently, the string format (used in the cells generated for the  from the DateTimeGroupDescription) is determined automatically based on the selected Step (Day, Month, etc.). Add an option to easily set custom string format.

At this moment, you can change the format only if you create a custom description and override a considerable amount of code. This is shown in the following SDK example: https://github.com/telerik/xaml-sdk/tree/master/PivotGrid/CustomDateTimeGroupDescription