Don't worry, we know that traffic in urban areas can have its surprises.
You can arrive at any time before your reservation and stay after the end without getting stuck. But be aware that any extra time you use before or after the reserved period will be charged at the current public rate and added to your initial bill.
The booking does not adapt to your arrival and departure times, there is no refund possible for an unused part of the pre-paid reservation.
It is a prepayment for a defined time slot, on a defined date. You benefit from the discount offered during the entire time slot, defined by the start and end time chosen in your initial reservation.