When creating a task, you can assign an optional deadline of a date and time. Simply click on the calendar icon when entering a task to select a date.

To add a time for when a task is due, you will need to ensure a date is added to the task (whether fixed or automatic - more information on adding automatic dates here). Once a date has been added, the clock icon will appear next to the task. Clicking on that will enable you to add a time for when a task is due.

Note: The time zone for the task due time is determined by the time zone in your account settings. To change this, simply go to Account > Settings and select the appropriate time zone from the drop down for your account.