```ruby
require 'ice_cube'
require 'sirius/teaching_time'

module Sirius
  class SemesterCalendar

    def initialize( teaching_period:, first_week_parity: )
      @teaching_period = teaching_period
      @first_week_parity = first_week_parity
    end

    def plan(teaching_time)
      week_offset = teaching_time.week_offset(@first_week_parity)

      scheduling_start = @teaching_period.starts_at + teaching_time.starts_at.seconds_since_midnight.seconds + week_offset
      event_duration = (teaching_time.ends_at - teaching_time.starts_at)

      event_schedule = IceCube::Schedule.new(scheduling_start, duration: event_duration)
      event_schedule.add_recurrence_rule to_recurrence_rule(teaching_time)
      event_schedule.all_occurrences.map { |event_start| Period.new(event_start.to_time, event_start + event_duration) }
    end

    private
    def to_recurrence_rule(teaching_time)
      week_frequency = 1 # every week by default
      week_frequency = 2 if teaching_time.parity != :both

      IceCube::Rule.weekly(week_frequency, :monday).day(teaching_time.day).until(@teaching_period.ends_at)
    end

  end
end

```

Fix scheduling issue with seconds_since_midnight on DST transition day.
user
```ruby
require 'ice_cube'
require 'sirius/teaching_time'

module Sirius
  class SemesterCalendar

    def initialize( teaching_period:, first_week_parity: )
      @teaching_period = teaching_period
      @first_week_parity = first_week_parity
    end

    def plan(teaching_time)
      week_offset = teaching_time.week_offset(@first_week_parity)
      scheduling_start = combine_date_with_time(@teaching_period.starts_at, teaching_time.starts_at) + week_offset
      event_duration = (teaching_time.ends_at - teaching_time.starts_at)

      event_schedule = IceCube::Schedule.new(scheduling_start, duration: event_duration)
      event_schedule.add_recurrence_rule to_recurrence_rule(teaching_time)
      event_schedule.all_occurrences.map { |event_start| Period.new(event_start.to_time, event_start + event_duration) }
    end

    private
    def to_recurrence_rule(teaching_time)
      week_frequency = 1 # every week by default
      week_frequency = 2 if teaching_time.parity != :both

      IceCube::Rule.weekly(week_frequency, :monday).day(teaching_time.day).until(@teaching_period.ends_at)
    end

    def combine_date_with_time(date, time)
      Time.new(date.year, date.month, date.day, time.hour, time.min, time.sec)
    end

  end
end

```
assistant