Getting a Calendar instant from ZonedDateTime is pretty straight-forward, provided you know that there exists a GregorianCalendar#from(ZonedDateTime) method. There was a discussion in Threeten-dev mail group, about why that method is not in Calendar class. Not a very deep discussion though.
However, there is no direct way to convert from an Instant to Calendar. You’ve to have an intermediate state for that:
Instant instant = Instant.now();
ZonedDateTime zdt = ZonedDateTime.ofInstant(instant, ZoneId.systemDefault());
Calendar cal1 = GregorianCalendar.from(zdt);
This is probably because, as evident from the table on this oracle tutorial, an Instant maps to Date rather than a Calendar. Similarly, a ZonedDateTime maps to a Calendar.