[Christian]

Start date is when the task becomes available to be worked on, no when it is due to be started working on.

Say you have to give the quarter's report to your boss. She wants it latest on Friday at 1am because that's when she will fly off to the board meeting.

Consequence: you would set the task "report for boss" to be DUE on Friday 1am. It is the latest time it has to be completed.

To compile the report you need input from your coworker. He has all the figures so working on the report makes absolutely no sense whatsoever until he gives you those numbers. He is on vacation until Tuesday, though.

Consequence: you can not start working on your task "report for boss" until you get the figures. It makes no sense that the task is even showing up in your list of tasks because you can not work on it anyways so it would only be in the way. You set your task to START on Tuesday afternoon because you know that by then you will have the numbers.

Start date basically means: here I noted down a task. Let's forget about it until day X. Then show it again so I know I can now work on it.