Willy's Freeware Agenda is a easy to use schedule application. Its interface has two panes each displaying a different date.

On startup the left pane displays today or the next available date. The following day is displayed in the right pane.

You can insert a new date or edit one in either pane.
The program doesn't allow you to insert a new record with an already existing date.

If you try to insert a new record with - or change a record's date to - a date for which a record already exists, the program beeps and the latter is displayed in the other pane. When you want to replace that record, just re-issue the command.