Hi Lan,
Thanks for your interest in Syncfusion products.
Apologies for the delay.
Please refer to the following sample link that shows how to set mininum/maximum summary value for datetime column using custom summary.
http://websamples.syncfusion.com/samples/kb/grid.windows/GGCMaxMinSummary/main.htmPlease let me know if you have any further concerns.
Regards,
Rajadurai