Subtasks and parent dates
Parent tasks in this app never get their own dates typed in — their whole date span comes from their subtasks.
Parent dates are derived, not typed
A parent task's start and due dates automatically stretch to cover its earliest-starting and latest-due subtask. You can't edit a parent's dates directly — the fields simply aren't editable on a task that has subtasks.
What this means in practice
To move a parent's end date, edit the subtask that's currently due last — the parent adjusts itself. This is also true in Bulk edit: a parent task keeps its derived dates and is skipped if you bulk-set dates across a selection that includes it, even though every other field you chose still applies.
Collapsing hides subtasks, not the calculation. Task buckets and, on the Gantt, individual parent tasks can be collapsed to hide their subtasks when you just want the overview — the parent's dates keep reflecting its subtasks either way.
Deleting a parent task
Deleting a parent task deletes all of its subtasks along with it — the confirmation dialog calls this out explicitly before you commit. This can't be undone.