Add timestamp and then file extension (xlsx) to a file and move to the archive Folder

str_archivePath location? its inside the project folder ?
try below
str_archivePath - assign in which folder you need to save and use below syntax
str_archivePath+Path.GetFileNameWithoutExtension(item.ToString)+DateTime.Now.ToString("MM-dd-yyyy")+".xlsx"

Regards,
Arivu