Hi Lan,
Sorry for the long delay in getting back to you.
You can get the maximum and minimum value of the date values by using a custom summary field. For Custom summary, the summary Base class is derived and the combine method is made use of to get the new summary value by comparing it with other summary values. Please refer the KB article available in the below link and let me know if it helps.
http://www.syncfusion.com/support/kb/grid/Default.aspx?ToDo=view&questId=358 Regards,
Subhasheela R