Interesting you bring best-guesses up! @NSmithSportEngland
There are proposals (changes to the Standard) for allowing the plannedDates start and end date values to be formatted as either:
-
YYYY-MM(year and month only, day unknown)
or -
YYYY(year only, month and day unknown)
as well as the currently accepted full-date (YYYY-MM-DD) and full date + time formatting currently accepted.
because these are quite often “uncertain” and no level of certainty beyond year or month may be expected in many cases.
See this thread for more discussion of that and other date format proposals.