Select the day using the calendar, then insert an entry as usual by using the Insert | New Time Entry menu item.  Expenses can also be entered the same way.

Because TT&E can add entries from other entries in addition to tasks, it depends on what item has focus. So to be able to do this, you must first highlight the date, and then select the task.

When in date view, this also means you can add a new entry by selecting an existing entry for a task and then selecting the Insert | New Time Entry item.

It is sometimes easier and less confusing to use the pop-up menus by right-clicking on the item after selecting a date.