@Lizzie the check-in time might be something that AirBnB could work with a small group of hosts to work out best option before a wider roll-out.
Would it help to have two non-time-specified choices - one "flexible" and one "anytime"? Or would that be too complicated...
"Flexible" could mean the host has a wide range of time that *could* work, but the host needs the guest to specify a time within that range - for example, it would let the host put in the outside time parameters (say 1pm to 10pm) but then requires the guest to select a time slot as part of booking. I think this is pretty much what @Ange2 has suggested and @Marzena4 seconded.
The existing check-in from (fill in blank) to (fill in blank) would remain an option for all hosts, and likely is the best fit for people hosting in their own home who work onsite or close by, or who do not work. It could also work fine for 24hr sites, they would simply enter a very wide range for "from" and "to".
"Anytime" could mean that the host has a lock box or other 24-hour access, and that the host doesn't care if the guest arrives at 3am. Hosts would still need to fill in the check-out time and some cleaning buffer info - eg, if 3 hrs is blocked off for cleaning, and if check out is 11am, "Anytime" check in would mean check-in is ok from 2pm through 10:59am the next day if there same day guest turnover, or 11am through 10:59am the next day if no guest turnover...or maybe make it ALWAYS 2pm to 10:59 so it's not as confusing?? but then "anytime" might not be the best word to use lol. This is where real world host and guest comments would help!! This "Anytime" arrival might best be paired with a no money back for first night in case of a no show guest. Or maybe this is all un-needed if guests read the requirements of "flexible" or time-specific check in parameters 😉