@Lorna170 As @Helen3 suggests, I tell my housekeeper to go right in and start cleaning. However - Airbnb has your back and this is something you can add to your rental agreement: Guest Terms, paragraph 2.3:
"If you stay past checkout, the Host has the right to make you leave in a manner consistent with applicable law, including by imposing reasonable overstay penalties."
Read the Terms of Service and you will see that Airbnb will back you in charging twice the nightly rate per day of overstaying. Drawing from that position, you can, as @Helen3 suggests, add a day before or a day after to the guests who are asking to arrive early or depart late. You can tell them that is the only way they can insure the property will be available exclusively to them for that time.
I have a guest coming this weekend who asked "How early can we check in?" I wrote back, "You can check in as early as 3 pm, Charlie!" 3 pm is, of course, the check-in time. Did this guest really think he could trick me into giving a different answer by the way he worded his question? Oy.