Time zone parsing from local time

operating_hour.rb

class OperatingHour
  class MissingTimeZoneError   < StandardError; end
  class MissingTimeStringError < StandardError; end

  attr_reader :time_string, :time_zone
  def initialize(opts={})
    @time_string = opts[:time_string]
    @time_zone   = opts[:time_zone]
  end

  def to_utc_time
    raise MissingTimeStringError unless @time_string
    raise MissingTimeZoneError unless @time_zone

    store_normal_time_zone
    set_time_zone

    time = parse_time_string
    time.utc
  ensure
    cleanup_time_zone
  end
private
  def parse_time_string
    Time.zone.parse(@time_string)
  end

  def store_normal_time_zone
    @normal_time_zone = Time.zone.name
  end

  def set_time_zone
    Time.zone = time_zone
  end

  def cleanup_time_zone
    Time.zone = @normal_time_zone if @normal_time_zone
  end

end