You can return your SIXT + car at any time. Returning a vehicle to the Sixt station selected in the application at, or prior to, the time and dated stated as the "due in" time on your applicable rental agreement. Return of the vehicle will be deemed to be a termination of the existing contract.
Please note that an early return before the end of a 30-day billing period does not lead to a proportional calculation of the respective time period. The contractually agreed monthly rate is always calculated for a full period of 30 days in advance, regardless of the time of return.