I originally posted this as a feature request, but I am now convinced it is really a bug. The current situation threatens to cause real problems for users within the way Task Coach is supposed to work right now.
My current example: I made a task, marked it planned for tomorrow. Then I made a subtask later. Because I have proposed date for planned start as today, there was no way to plan the subtask for tomorrow also without messing up the composite task, unless I were to first move the subtask out from the parent. The problem is, the date and time are not editable until I click the check to activate planned date. Activating it sets it to today, and this forces the composite task's date to match. Thus, I have to go back and fix the composite task. But all I wanted was for both tasks to be marked for tomorrow!
This is a serious problem if someone makes a mistake when editing a subtask's dates. Currently, changing a subtask's dates forces a change in the parent's dates, but this is not necessarily shown to the user. So if the user makes a big mistake, say the wrong year, and then fixes it, they may not realize the need to fix the parent task's dates! And sometimes there is no way to remember what the intended date originally was!
Partial fix idea: make proposed dates and times editable. In other words, allow the dates and times to be edited even though the date in question has not been checked as active. This way, I could at least get the dates I want and then check them afterwards. This could still result in accidentally messing things up, but it would at least function.
Better fix idea:
A. Override the default preferences for proposed dates when making a new subtask. All unmarked dates for subtasks should have proposed dates that match the parent task. I prefer this solution as the most seamless. It is often the case that users will want subtasks to have dates that match their parents.
B. Have some conflict dialog come up whenever a subtask (whether one being moved or one being changed) has dates that conflict with the parent task. Allow the user to choose some resolution so that dates don't get forcibly changed without intention. The dialog should have three options: "adjust the parent task to match the new date", "adjust the subtask's dates to match the parent", and "undo this action" (i.e. undo the date change that was set or undo the moving of the subtask into the parent.
I'm happy to discuss more how to best solve this, but I really think this needs to be solved soon.