Add this as the expression for a pane column:
FormatDate([Date Imported,0], Year)\/
FormatDate([Date Imported,0], Month)/
&datatype=[list]
The backslash is the list hierarchy separator for levels in a pane column (e.g. B will be indented when A\B is presented). It is enabled by forcing the datatype to list: &datatype=[list]. Unfortunately, only one datatype can be specified, so you'll lose the alphabetical month sorting. You could use numeric sorting.
Edit: You can set the field's Sort order to be Date Imported. This should sort nicely.