Recurring dates + (nn) days -- something we all need in real life that no calendar/task manager handles
This is something that NO task or calendar manager seems to handle and yet is a common situation. It's best explained by an example.
A lot of places in the US have the day after Thanksgiving as a day off. The date of Thanksgiving is defined by being the 4th Thursday in November. However, the day after can either be the 3rd or 4th Friday in November. So really, the date of that day off is the 4th Thursday in November + 1 day (for example, a month where November 1 falls on a Friday will do that).
A use case that also affects me is that some vendors have a given day they issue patches (Microsoft has "Patch Tuesday" as the second Tuesday of every month). We have a schedule at work of how patches must be applied so they go onto non-production systems on Wednesday and Thursday and Production systems on Friday. Again, there's no good way to handle that reliably.
I would love it of Todoist were the first Task Manager in history to handle that, like "every second Tuesday + 2 days" for non-production patching task, and "every second Tuesday + 3 days" for our production patching task.