Refactor tests so order doesn't matter.
ActiveSupport::TimeWithZone#to_r is broken, which
causes Time#at to return incorrect results.
Explicitly call #utc before passing to Time#at.

https://github.com/rails/rails/pull/9403

Also added some tests to ensure correct time zones
are returned.
Refactor initial parsing of frozen time, add a test
for freezing an ActiveSupport::TimeWithZone.
fix calculations with nanoseconds (close #70)