Timelines can display adornments, if the adornment has a $StartDate and $Enddate. These adornments are in fact shared with the map view of the same container, and use their colour, border, etc., values. The 'smart' elements of any such shared adornments (querying, etc.) only work in map view and so are ignored in timeline view. Adornments in maps of child/descendant containers are not shown. Thus a given map will always show all its adornments but the timeline of the same item will only show adornments with start/end dates.
Adornments are drawn from the top of the timeline view to the bottom of the bottom timeline band. Adornment staking order (if overlapping) is as per map view), though this can be still be adjusted in timeline view using the same controls as for map view.
Dragging a selected adornment right or left will move it and adjust $StartDate/$EndDate. Dragging the left of right drag-handle of a selected adornment allow separate adjustment of $StartDate/$EndDate.
As the Create Adornment dialog has no way to set $StartDate or $EndDate, it is advisable to create new timeline adornments in the map view of the timeline's container, even if said map view is not used for any other purpose.
Although the timeline view context menu allows you to add an adornment, the Create dialog does offer a means to set starting and ending dates. It is better to create adornments in map view, then use their Info view to set $StartDate and $EndDate. Setting the latter will then make the adornment show in timeline view.