OffsetDateTime to milliseconds

I would just convert the OffsetDateTime to an Instant and then use toEpochMilli:

long millis = book.getInteractionDuration().getStartTimeStamp().toInstant().toEpochMilli();

Unlike toEpochSecond(), this approach won't lose any more precision than is inherent in wanting milliseconds rather than nanoseconds.

Try this:

long millis = offsetDateTime.toInstant().toEpochMilli();