Tasks in the Timeline view take on an a certain appearance based upon their properties and state.
Map Branches
Branches that exist in your Map view, but which don't have any task properties set against them, will still appear in the Timeline (unless hidden by a filter). These branches will display as a lighter blue bar diagonally striped with a slightly darker blue.
Since these branches don't have a date against them, they'll default to that days date.
Tasks
Unscheduled
Tasks will display in one of two ways depending upon whether they are scheduled (have date properties against them) or unscheduled.
Unscheduled tasks are those tasks with properties such as assignee or status set against them but no start or end date set yet.
An unscheduled task will show in the Timeline as a solid light blue taskbar. Similarly to map branches (above) these tasks will default to that days date until date properties are set against it.
Scheduled
Tasks with date properties set will be classed as scheduled and will display most prominently in the Timeline with a solid dark blue taskbar
Completed
Tasks which have been marked as complete will still show on your Timeline (unless otherwise hidden by a filter) as a solid green bar.
Summaries
Summaries appear in the Timeline if a branch has subtasks against it but no end date set against the task itself. The duration will then roll up based upon the further end date set against it's sub tasks and the branch will take on the appearance of a summary bar in the Timeline.
