How can I set a start date?
I have a task with a due date, but it can only start 3 weeks before the due date.
How can I set a start date (like in Outlook) so that I can then sort my tasks by start date?
Unfortunately, it's not possible to set start dates in Todoist, you can only add one date to a task (the due date), but we may consider adding such option in the future and you're welcome to vote for this request in our Votebox: http://todoist.com/Vote/showProposal/415/
For now, as a workaround, you could try to consider due dates as start dates and use labels as deadlines so, for example, the task's due date can be set to the time when you must start working on it and you could put a label showing the deadline of when this task must be completed. You could type @20/4 within the task name to do so.
Also, with Todoist Premium, you could use the due date as a deadline and then set an email, SMS or mobile push reminder to 3 weeks before this deadline to remind you about the start date.
This is an interesting solution. Another could be to put an item's due at it's start time, and a child item as it's deadline. Eg.
☐ Open your test booklets > Mar 18 @ 1:00pm
☐ Final problem due > Mar 20 @ 2:00pm
Then you would get a reminder of when you can start on a project, continue to be nagged if you choose by leaving it unchecked, and also get a final reminder when whatever is due, that can be dismissed when you have completed it.