When adding a date to an item, have a button that says, "Display X days in advance." The user can then put in the number of days. For example, if I have an event that is due February 10 days, but will take me 3 days to do, I can set the event due date to Feb 10, but set "Display X days in advance" to 3. Now, the item will show up in my Today section in agenda view on February 7, with a due date of Feb 10. Also, it will stay in the Today view on subsequent days instead of showing as overdue, until after Feb 10. (It may also be helpful to have a small icon next to these items or to have them be a dark gray text instead of black so that it's clear that this is a notification of something upcoming, and not actually due today.)