Currently MinValue and MaxValue properties of the Axes accepts only decimal, so that when the axes are of date type, Min and Max dates cannot be set.

The workaround is to obtain the min and max values from the datasource field used for populating XAxis labels and set these values explicitly to the XAxis through the chartObject. For example:
<script type="text/javascript">
    function pageLoad() {
        //Get reference to the chart
        var chart = $find("<%=RadHtmlChart1.ClientID%>");
  
        //Obtain charts datasource
        var currDataSource = eval(chart.get_dataSourceJSON());
  
         //Get minValue for the datasource field used for populating XAxis Labels. In this example the field is named myDataSourceField
        var minValue = currDataSource[0].myDataSourceField;
  
        //Get maxValue for the datasource field used for populating XAxis Labels. In this example the field is named myDataSourceField
        var maxValue = currDataSource[currDataSource.length - 1].myDataSourceField;
  
        //Reference the chartObject's min and max properties
        chart._chartObject.options.xAxis.min = minValue;
        chart._chartObject.options.xAxis.max = maxValue;
  
        //Repaint the chart
        chart.repaint();
    }
</script>