What about the Durations and Intervals? The "Week" format is great for planning purposes, because it allows a granularity between days and months with another

What about the Durations and Intervals? The "Week" format is great for planning purposes, because it allows a granularity between days and months with another whole number per year. Saying that a task will begin 2021-W08 and then defining the start of the week (ISO8601 says its a Monday), tells you exactly when it will start. Saying the task is 2021-W08/P4D tells you exactly when it starts and how long it will take. Or P4D/2021-02-25 says the same thing but in terms of when it will end. They're just as unambigious and just as useful.