<Application x:Class="WpfApp1.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:WpfApp1" 
             xmlns:telerikDialogs="clr-namespace:Telerik.Windows.Controls.RichTextBoxUI.Dialogs;assembly=Telerik.Windows.Controls.RichTextBox"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/System.Windows.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.ImageEditor.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.Navigation.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.Input.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.RibbonView.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.RichTextBox.xaml"/>
            </ResourceDictionary.MergedDictionaries>            
            <Style TargetType="telerikDialogs:LineNumberingDialog" BasedOn="{StaticResource RadWindowStyle}"/>
            <Style TargetType="telerikDialogs:LineNumberingDistanceNumericUpDown" BasedOn="{StaticResource RadNumericUpDownStyle}"/>
        </ResourceDictionary>
    </Application.Resources>
</Application>

  public class MyRenderer : SpreadStreamExportRenderer
  {
      public override void SetCellValue(DataType dataType, object value)
      {
          DateTime date;
          if (value != null && DateTime.TryParse(value.ToString(), out date))
          {
              base.SetCellValue(DataType.DateTime, date);
              var cell = this.GetCell() as ICellExporter;
              cell.SetFormat(new SpreadCellFormat() { NumberFormat = "yyyy-MM-dd HH:mm:ss" });
              return;
          }
          base.SetCellValue(dataType, value);
      }
  }

 spreadStreamExport.RunExportAsync(FILENAME, new MyRenderer(), options);