How to Enter New Tasks in the Outlook 2013 Tasks Module
(Optional) Click the date box next to the Reminder check box and enter the date when you want to be reminded.
If you entered a due date, Outlook has already entered that date in the Reminder box. You can enter any date you want. If you choose a date in the past, Outlook lets you know that it won’t be setting a reminder. If you open the scroll-down menu by clicking the icon on the right of the date box, a calendar appears. You can click the date you desire in the calendar.