To reproduce:

public class Item
{
    public int Id { get; set; }

    public string Name { get; set; }

    public DateTime Date { get; set; }

    public Item(int id, string name, DateTime date)
    {
        this.Id = id;
        this.Name = name;
        this.Date = date;
    }
}
public Form1()
{
    InitializeComponent();
    List<Item> items = new List<Item>();
    for (int i = 0; i < 10; i++)
    {
        items.Add(new Item(i,"Item" + i,DateTime.Now.AddHours(i)));
    }
    this.radGridView1.DataSource = items;
    this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
}

private void Form1_Load(object sender, EventArgs e)
{
    System.Threading.Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo("en-GB");
    this.radGridView1.Columns["Date"].ExcelExportFormatString = "M/d/yyyy h:mm tt";
    this.radGridView1.Columns["Date"].ExcelExportType = DisplayFormatType.Custom;

    GridViewSpreadExport spreadExporter = new GridViewSpreadExport(this.radGridView1);
    spreadExporter.ExportVisualSettings = true;
    SpreadExportRenderer exportRenderer = new SpreadExportRenderer();
    string fileName = @"..\..\exportedFile" + DateTime.Now.ToLongTimeString().Replace(":", "_") + ".xlsx";
    spreadExporter.RunExport(fileName, exportRenderer);
    Process.Start(fileName);
}