This doesn't work that way you want. Default weekly calendar show only time intervals that are set in object settings.

If you have 14 hours per day for a week then it shows 14 hours per day. If you have 18 hours per day for weekends then it shows 18 hours per day in weekends. We also do not use any grid to display weekly calendar and time intervals are displayed in every box (table cell). I don't want to say, it is bad idea, but our default calendar has not this bug.

You have problem only with your modifies. If you set time intervals from the first day only then it is clear that you get this type of error. You are printing other days without any comparing of day time intervals with time intervals of table rows.

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Okay, so maybe I just didn't understand the way the reservation system works.

Where can I set this global time interval? Because as of now I defined all my time intervals in the "Prices" tab of the "Object". So I defined a reservation persion during weekdays from 17:00-00:00, and on weekends it is from 00:00-00:00 (e.g. 24hrs). But if it is possible to tell the system, that there is always a 24hrs interval, but reservations are only possible during the defined time period, then this would be the solution for my problem.

Well,

because you are using only 60 minutes as time intervals, you can try to set Fix Shedule in Reservation types to maximal and minimal time and also do not forget to set Min. interval [min] to 60. Is that helpful for your type of calendar?

Yes, I did that. But this doesn't affect the problem. I have attached the new settings together with the template that comes with com_booking:

administrator/components/com_booking/helpers/booking.php - line 469
$sheduleTo = BookingHelper::timeToFloat($subject->shedule_to);

Modify it to:
$sheduleTo = (BookingHelper::timeToFloat($subject->shedule_to) == 0.0 ? 24.0 : BookingHelper::timeToFloat($subject->shedule_to));

Very good! Thanks a lot. This looks promising, and seems to solve the issue. I will have to adapt a bit the template, but it's okay.
Thanks