Refactored start_date validation/modification

user.rb

class User
  MIN_DATES = {
    'student'   => MINIMUM_STUDENT_START_DATE,
    'professor' => MINIMUM_PROFESSOR_START_DATE
  }
  
  def handle_invalid_start_date
    return unless valid_type?(type)
    
    self.start_date = MIN_DATES[type] if start_date < MIN_DATES[type]
  end
end