How to get only the year and date in the Gantt Chart Tooltip template? The DataAttribute is not a valid date string.

<TooltipTemplate>
     <h4>@(((TooltipTemplateContext)context).Title)</h4>
     <h5>Start: @(((TooltipTemplateContext)context).DataAttributes["start"])</h5>
     <h5>End: @(((TooltipTemplateContext)context).DataAttributes["end"])</h5>

</TooltipTemplate>