Hi

 The way to do custom formatted value is as follow:

1.       Create new cell style as resource
<Style TargetType="igPivot:PivotCellControl" x:Key="NewCellStyle">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="igPivot:PivotCellControl">
                <Grid>

                   …
                   <TextBlock Text="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Cell.Data,Converter={StaticResource CellValueFormat}}" />

                   …
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

 2.         Add converter class to your project. In convert method apply your custom formatting logic.

 using System.Windows.Data;
using Infragistics.Olap.Data;
    public class FormatValueConverter : IValueConverter
    {
        #region IValueConverter Members         public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            ICell cell = value as ICell;
            if (cell != null)
            {
                //apply here your formatting string
                return new DateTime(long.Parse(cell.Value.ToString())).ToString("hh:mm:ss"); 
            }
            return "";
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            return "";
        }
       #endregion
    }

3.       Apply the new style to pivot cell

<ig:XamPivotGrid Grid.Column="0" Name="pivotGrid" DataSource="{StaticResource pivotGridDataSource}" CellStyle="{StaticResource NewCellStyle }"/>