[C#]
private void dataGrid_NodeCellChanging(object sender, NodeBase node, int index, ref object value)
{  
  FlyGrid flyGrid = sender as FlyGrid;
  Column col = flyGrid.Columns.FieldMapColumnFromIndex(index);
  if (col is DateTimeColumn)
  {
    try
    {
      //validate data
      DateTime.Parse(value as string);
    }
    catch
    {          
      value = col.GetValue(node);//revert to current value;
      MessageBox(\"incorrect data\");
    }
  }
}