custom time stamp causing error

[ZaynJarvis]

FirebaseError: Function DocumentReference.set() called with invalid data. Unsupported field value: a custom Timestamp object (found in field startTime)

I am encountering this error from last week. I am not sure how to debug this, is it the case that firebase disallowing adding custom timestamps?
I tries to use Timestamp.fromMillis(event.startTime), and Timestamp.fromDate(event.startTime),
Doesn't seem to work in both case.

[ZaynJarvis]

Still having this unexpected error. Help wanted.