Quick answer
When you book a single working day, choose Full day, Morning ½ or Afternoon ½. For a longer range you can start after lunch on the first day, or be back after lunch on the last. A half day counts as 0.5.
Find it under
Book a single half day
Open the booking sheet with the Book button and set the same date for Starts and Ends. A Length choice appears: pick Morning ½ or Afternoon ½. It takes 0.5 days from the allowance.
Half days within a longer break
For a range across several days, use the half-day options to start after lunch on the first day or be back after lunch on the last, so those days count as a half.
How half days show
A half day appears on the calendar and Team view with an AM or PM tag, so it is clear at a glance which part of the day is booked.