Hello.
The bookings for specific timeslots possible in the Booking Calendar Free version. But I recommend to check the Booking Calendar Business Small version, where possible to set the start time selection and duration of time selection, as in your example: 30-60-90-120, etc minutes. In Booking Calendar Free version possible to make bookings for specific times only for specific restricted time-slots.
You can check how to add and configure timeslots in the Booking Calendar Free version here https://youtu.be/-pOTMiyp6Q8?t=28s
In Booking Calendar Business Small or higher versions possible to use different times fields for the bookings.
Please note if you will make the booking for the specific timeslot, this timeslot become unavailable for the other visitors for that selected date in current specific calendar.
Please watch more in this video: https://wpbookingcalendar.com/help/booking-calendar-video-timeslots/
You can configure the timeslots selections on the Booking > Settings > Form page. Please use the “shortcode generator” on the right side of that page for the correct generation of the shortcode.
You can use one”timerange” (timeslots) or “start time” and”end time” selections or”starttime” and “duration”of time selections shortcodes or even start time and end time entering shortcodes. Please read more about the booking form fields configuration here https://wpbookingcalendar.com/help/booking-form-fields/
You can test the bookings for the specific time-slots in action on the live demo of Business Small version (which is configured by default for the bookings of specific time-slots) here: https://bs.wpbookingcalendar.com/ Other live demo you will need to reconfigure.
Kind Regards.