There's a whole pile of relative time modifies you can use in the search, for instance
index=ha earliest=-1d+8h@h latest=@d+8h
That searches my index "ha" for events where the earliest _time matches
-1d = go back one day to yesterday
+8h = add 8 hours to that (so 8 hours after the beginning of yesterday)
@h = "snap" to the hour 8:00:00 instead of using current minutes/seconds like 8:18:35
and the latest _time is no later than more or less the same as the above, only instead of going back a day, it just takes the snap to beginning of current day and adds 8 hours.
You can put these in the time picker, too!
Click your time picker, then at the bottom click on "Advanced". If you paste the -1d+8h or whatever into there for the earliest/latest times, you can even see what it turns into.
Also see these docs for Time Modifiers.
