We would suggest you to review this online example here
that demonstrates how you can use DateTime data with the numerical axes of the RadChart control. The Appearance.ValueFormat / CustomFormat properties do not specify a custom format for parsing strings into doubles -- instead you need to pass a valid double value to the chart control via DateTime.ToOADate( ) method, then you can use the ValueFormat / CustomFormat properties to format the display of the double values that actually are your DateTime data values.
Hope this helps.
All the best,
the Telerik team