If you need a schedule for a 24/7 business, then:

a) unlike a 'normal' schedule when you can set the End Time to the actual end time (eg 17:00 for a 9am-5pm business), you cannot set the end time to 24:00 and 23:59 is the best you can do

b) when you look at the 24hour schedule the last slot is missing.  ie if you have a 30 minute slot size, the the last slot for the day is 23:00; if you have a 15 minute slot size, it is 23:30

Note that appointments across the midnight boundary (eg 23:00 to 01:00 the next day) work OK- the only problem is the missing last slot.

However, appointments up to the midnight boundary do have a problem.  If you specify say 23:00 to 23:59 then I think it works (but the last slot is missing).  However, if you specify 23:00 to 00:00 the next day, then the appointment will also occupy the first slot (ie 00:00 to 00:15) on the next day.

Regards, Tim