My problem with this kind of calendar reminder is, if I don't act on it immediately, it's gone forever and I notice it, at best, next time it's "time" to change the furnace filter.

It would be super helpful to me to have a calendar reminder that succinctly said "you said you wanted to do this every 6 months, and now it's 23 days overdue" (and then when I finally do the thing, schedule the next one for 6 months after that day, not 6 months after I was supposed to do it in the first place).

Various "habit tracker" apps do this pretty well, but they don't fit in with my calendar.

> "you said you wanted to do this every 6 months, and now it's 23 days overdue" (and then when I finally do the thing, schedule the next one for 6 months after that day, not 6 months after I was supposed to do it in the first place).

org-mode with its agenda does this beautifully. But then that only really runs in emacs.

Usually I get the notification for this thing, and I never get rid of the notification until I act on it

This gets close but I don’t think it will then allow you to easily see which of these periodic tasks you can work when you have a few hours to spare today.

Yeah, that's fair, I usually don't look at them in a list. I know they show up in Google Calendar, so maybe you can view them their? Regardless, this is a great idea for a Todo app feature.