Data from the assignment tracker (your spreadsheet) is stored on a server in Toronto, Canada. The tracker only accesses the courses you're enrolled in, your assignments and their respective due dates. The tracker does not access any of your personal information, your grades, etc. For verifiability, the tracker is open-source. Anyone can view the code here, and see exactly what data is being accessed. All data stored is deleted off the server at the end of each semester.