kotlin-flow-extensions VS kotlinx-datetime

I don't know, why I'm downvoted here, but it is very relevant if the library provide the operators you need or if you have to implement them yourself. If Kotlin Flow is enough, then use it. From experience you don't have to go to far to miss operators from Reactor or RxJava. Maybe some day additional libraries like https://github.com/akarnokd/kotlin-flow-extensions will fill the gap, but for the time being Reactor or RxJava offer a lot more for (not trivial) real world use cases.

Why do you want to keep Flow such a small library? It could be an attractive alternative to RxJava or Reactor, but it misses many useful operators today. On the other hand it brings a lot more operatores than would be required as a foundation for other frameworks (likeKotlin Flow Extensions). From my point of view Flow stays in the middleground with no good reason.

Kotlin multiplatform types often come with their own serializers. https://github.com/Kotlin/kotlinx-datetime and https://github.com/hfhbd/kotlinx-uuid do, for example.

But we do have such plans for our kotlinx-datetime library and expect to provide at least a partial solution during the course of the year

It depends, time has a complex and native bound legacy. But I bet you want this https://github.com/Kotlin/kotlinx-datetime and DateTimePeriod.