def period_for(time)
      raise ArgumentError, 'time must be specified' unless time
      timestamp = Timestamp.for(time)
      raise ArgumentError, 'time must have a specified utc_offset' unless timestamp.utc_offset
      info.period_for(timestamp)
    end
def period_for(time)
      raise ArgumentError, 'time must be specified' unless time
      timestamp = Timestamp.for(time)
      raise ArgumentError, 'time must have a specified utc_offset' unless timestamp.utc_offset
      info.period_for(timestamp)
    end
(see Timezone#period_for)