Hi Ali,
Thank you for your interest in syncfusion products.
You can show the summary of the time by using the CustomSummary method. Please refer to the following our sample browser path whis is illustrates the same.
{Installed Path}\Syncfusion\EssentialStudio\{version}\Windows\Grid.Grouping.Windows\Samples\2.0\Calculate Summary\Custom Summary Demo
Let me know if you have any further concerns.
Regards,
Ragamathullah B.