If you wish to enable the clock on your calendar, you will need to turn it on under the Clock dropdown. Once enabled, you will be able to set how many hours there are in a day, and how many minutes there are in an hour, as well as some visual options for the clock.
This will enable the clock, allowing you to edit it.
You might want to have a clock, but not deal with the actual visualization of the clock. Disabling this will hide the clock.
This sets how many hours per day your calendar will have.
This input sets how many minutes the hours on your calendar's will have.
This setting will rotate the face of the clock, so that midnight doesn't have to be on the lower part of the clock.
Increasing this number will show only every nth number. So if you set it to 1, the clock will only display every even number (removes one number). Setting it to 2 will skip 2 numbers, showing 0, 3, 6, 9 and so on.