At the moment for different prices during different times you have to create 2 separate cars with different availability and add the different price in the Fixed fare route - it will show both cars.
The customer at the front end will not notice any difference as the system will only show the car that is available for the selected time by the customer.

Note: When you use this method you have to take into account that after midnight a new day starts. If you need to create different price for 8 pm (20:00) to 6 am (06:00) time period you will actually have to create 2 separate cars with different availability to cover this time period: 1. 8 pm (20:00) to 11:55 pm (23:55) and 2. 12 am (00:00) to 6 am (06:00). Select the option for each car "Use for Fixed fare routes: Yes" and they all will be available in your Fixed fare route edit screen where you can set a different price for each car.