I feel like the 3rd party API should have had some error checking, although that might have strayed too far into a client’s business logic.
If it is an API of incidents, that suggests past incidents. And the whole “never trust user data” kinda implies they should throw an error if you request information about a tinerange in the future.
I guess, not throwing an error does allow the 3rd party to “schedule” an incident in the future, eg planned maintenance/downtime.

But then, that isn’t separation of concerns. Ideally those endpoint would be separate. One for planned hypothetical incidents and one for historical concrete incidents.

It’s definitely an odd scenario where you are taking your trusted data (from your systems and your database), then having to validate it.