Timeline can only be used when the command "--due" is used. When you add a new task with the "--due", success message is shown but no new tasks is shown on the timeline.
for e.g. the command : add_task task333 "15/11/2019 12:30" does not work, but the command add_task task333 --due "15/11/2019 12:30" works for adding into timeline.
add_task task333 "15/11/2019 12:30" is redundant

After adding the new task, nothing shows up on the timeline

[original: nus-cs2103-AY1920S1/pe-interim#2553]