This is a known problem in the RadTimeline control. You can check the workaround provided in the feedback item
for a solution that you can use.
Regarding your other questions:
"The MouseLeftButtonDown event gets triggered but the DataItem of sender is null." - Based on the provided information, I am not quite sure which is the sender in your case. In case you need some specific information regarding obtaining information about timeline items, please provide more details regarding your setup and expected result.
"Is it possible to trigger a event and get date and time when user clicks on the year or month tabs. " - You can use the ConvertPointToDateTime method
of the RadTimeline control to convert a point to its DateTime representation on the timeline of the control.
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