rspec; How to test this function?

ImageManager.check_enable_time

def check_enable_time
   # get current time
   now_time = Time.now
   # UTC to JST convestion JST = UTC + 9 hours
   hour = now_time.in_time_zone("Asia/Tokyo").hour
   (hour != 23) ? true : false

end

This function return true if current time in JST != 23 else returns false.

I want to test this function.

What I tried:

 describe ImageManager do
  describe "Test check_enable_time() function" do
    context "When current time in JST != 23" do
      it 'should return true' do
        image_manager = ImageManager.new
        result = image_manager.check_enable_time
        result.should eql(true)
      end
    end
  end
end

How do make now_time.in_time_zone("Asia/Tokyo").hour return 23 and other than 23?

Please help I am new to rails and rspec.