Try this in your xtzColumns.cs for the given columns:

[SortOrder(1, descending: true)] public DateTime TimeStamp { get; set; }

Note: The Integer 1 here is for hierarchy of sorting by Multiple Columns.

Next you must make sure that sorting is not being handled client side in your xyzGrid.ts file by code similar to this example : getDefaultSortBy().

The Northwind OrderColumn.cs file has an example to look into.

Info on this page was taken from: Issue 1393

Information for custom handling can be found here: Custom List Ordering