class Time

def minus_with_coercion(other)

are coerced into values that Time#- will recognize
We're layering on additional behavior so that ActiveSupport::TimeWithZone instances
Time#- can also be used to determine the number of seconds between two Time instances. 
def minus_with_coercion(other)
  other = other.comparable_time if other.respond_to?(:comparable_time)
  other.is_a?(DateTime) ? to_f - other.to_f : minus_without_coercion(other)
end