Thank you for your reply.
I have updated the first one with 1d. I can probably set it to 1 year for the other one, as it should make an observation at least once a year. This might be the best solution now, but it would be nice to have some sort of flag to indicate sporadic observations. That is what I was looking for anyways. But with good guidance the solution with the 'Impossible schedule' might be good enough.