In order to set DateTime as value, please, use DateTime.ToOADate()
. Also, note, that you need to set
RadChart.PlotArea.XAxis.Appearance.ValueFormat = Telerik.Charting.Styles.ChartValueFormat.LongDate;
You are correctly setting the tooltip and it should be working. Please, provide more details on thie issue so that we can further assist you.
the Telerik team