Error using validation hooks with before_save callback

# This will throw "Schedule can't be blank" error as any changes made in before_save callback somehow gets lost
class Assessment < ActiveRecord
  validates_presence_of :schedule
  
  attr_accessor :date, :time
  before_save :set_schedule_from_date_and_time

  private

  def set_schedule_from_date_and_time
    if self.date && self.time
      self.schedule = Time.parse("#{date} #{time}")
    end
  end

# The trick is to use before_validation
  before_validation :set_schedule_from_date_and_time