Which field are you using in order by clause or grid sorting expressions? Is it the original date field there or the formatted one? I suggest that you use the original field in this case.
If you could send us a sample project illustrating the described behavior can help us get to the source of the issue and find a resolution.
Additionally, you can review the below topics for more information about grid sorting and grouping:
Looking forward your reply,
the Telerik team
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