Forgot to mention this: As my first reservation was from 12:00 and 11 days later returning at 18:00 I actually pays for 1 week + 5 days as SIXT count's in full days.
Now the change I want to make is to pick-up the car at 18:00 the day before - which means that rather than having the day for 11 days and 6 hours I would have the car for 12 days - which is exactly what I am paying for; so the odd thing that the period I would pay for would not change.