Hi Giang,
Thank you for the update.
Regarding the "Day360 functionality", on our further analysis it is not a bug, it is default behavior for Day360 . Please refer the below documentation link from from below to get a information about the "DAY360" function.
http://help.syncfusion.com/UG/Reporting/Calculate/ASP.NET/documents/4737days360.htmPlease let me know if you have any other concerns.
Regards,
Lingaraj S.