`$startDate` and `$endDate` could be accepted as `instanceof \DateTimeImmutable`

[Michal Borychowski]

IMHO $startDate and $endDate could/should be accepted as well as the instances of DateTimeImmutable.

PS. Very easy to implement at dateTimeObject() but was not sure about occursOn() and that's why I have not prepared the PR.

[Tom Planer]

Take a look at the last commit: 84d280a