I don't see a way to set these fields.
I'm pretty regular if it can be set by check/checkout day of the week or even by the season. Would be nice to be able to override it individually, too.
Those are booking field codes. The value that fills them comes from the check in/out times found under the Booking > Dates tab. By default, bookings are set to "standard" for both check in and out. You can adjust the times to something else. There are also email templates can be sent out (to you, 3rd party alerts, and the guest) when the check in/out times change.
This blog post talks more about early/late check in/out times on bookings:
Hope this helps,