You can change this timeframe, or you can also choose to have the whole block of time be a one-time slot by clicking Slots with duration and choosing Single slot.

Make sure you are viewing your calendar in a web browser on a computer and not using the Calendar mobile app.

You can only create appointment slots while viewing your calendar in the Day or Week views. More detailed instructions can be found on Google's Documentation page. For example, professors can invite their students to reserve time during office hours each week. You can create blocks of time on your calendar designed to let others reserve an appointment with you. These are called Appointment Slots in Google Calendar.