Hi Marvin! I think I might’ve replied to you by email as well, if I’m remembering correctly. Beeminder is usually all about doing things consistently over time – more like “brushing my teeth every day” and “working for an hour a day on my dissertation” than “finish this one task by [date]”. The expectation is that you’ll make incremental progress every day… and then we automatically use integers until you use a non-integer datapoint, so as soon as any amount is due, +1 is due!
You can set goals up with end dates so that they automatically end on a certain date, but the goal would slope up toward that. I guess you could create a goal with a rate of 1/year, start with 30 days flat spot, and then use the road.beeminder.com tool to tweak things so it’s totally flat until the date you want to have completed the task… but of course that’s not really the sort of thing Beeminder is meant for. We also can’t check whether a specific Todoist task is done, though if you put it in its own project and beeminded that project, I guess that’d work.
There is a service that would work pretty much the way you want, though, and that’s Taskratchet. It can’t tell if a task on Todoist has been done, so you’d have to do the check-off manually… but it’s designed for single tasks. If you have iOS, GTBee is a Beeminder-linked app that can also do single-time tasks – we’re not actively developing for it, but at present it still works (as far as I know) and might also be useful.
Personally, my rec would be Taskratchet.