InfoPath is required especially if they are opening a new form from SharePoint. There are ways to get around it. However, without Infopath, its a hack job and too complex for basic users.
1) If they are opening an existing form (xml), then they may get away with some type of xml editor.
2) If they launch a brand new timesheet by double clicking the .xsn file, they can go into the .xsn using winzip, modify the manifest file and zipping it back to a .xsn file.
Don't grow old regretting the things you didn't do.