Use java.time.Instant instead of ZonedDateTime

[sappenin]

java.time.Instant will always be created using a fixed clock based off the epoch in UTC. We should consider moving to this object in order to enforce a consistent Timezone in our library.

For library users who want something more time-zone centric, Instant's can be trivially converted to ZonedDateTime or other java.time classes, like so:

Instant instant = Instant.now();
ZonedDateTime zdt = instant.atZone(ZoneId.of("PST"));