Calendaring and clients that postpone the meetings

As a real estate, I have many meeting with clients. I schedule them mostly in the afternoon because these meetings don’t require from me much focus. I do my focus time in the mornings.
However, often they call to postpone it. Then, I have a gap in my schedule that’s hard to fill because my brain gets straight to the thought “Ok, I have time off then.”
The result is I have to find time for this meeting instead of other things I planned. It’s so hard to force myself to work in these gaps.
How could I prevent this from happening? When a client postpones, it usually has to be 1-2 days later.