If so, how would both parties determine when it was an hour before sunrise, since the sun rises at different times throughout the year - or was it more of a hit-or-miss kind of thing?
Follow-up question: how would they avoid oversleeping?
"How did people wake up before alarm clocks?"
FAQ: Waking up without alarm clocks for more general timey-wimey answers.
Hope some of those help!
The time at which the sun rises varies slowly and predictably. It has been understood for a very long time. The great pyramids of giza demonstrate a clear understanding of how the suns movements in the sky vary with the seasons, as do many other ancient civilizations. Therefore even before mechanical clocks, someone could easily have taken the steps to predict even first light, by using either candles, or by tracking a fixed flow rate (water clock, hourglass).
I think the best answer to your question is probably 'that's not an unspecific time.'
Just want to add to be sure I'm not too dismissive, that the question of when exactly someone would have first been able to measure the duration of the night and use that knowledge to meet someone an hour before sunrise would be better suited to an archaeologist than a historian, since the necessary technology of a bowl with a small hole in it long predates the writing required for historical record.
I think a likely hypothetical first pre-dawn appointment might take us to a hunter gatherer society. Some lucky proto-shaman is about to encounter bad luck, that turns out to be good luck. He has just dropped one of the clay pots his tribe uses to store water, and by sheer luck, or clever curiosity he notices that it takes almost exactly the duration of the night to drain through the crack. He is a clever man, and he has an idea. No one else knows exactly when the sun will rise, so he will make a spectacle of predicting the first light exactly, or close enough that he can appear to conjure it with a bit of theater. Then he will quickly become the first person to notice that the length of days change as the demons of cold crept in to steal the power from his secret holy bauble.
Scenes like that one are possible from the moment early man develops a vessel to carry water (or sand, or something else, though water seems most likely), possibly even an animal skin, which is something even pre-human genus of homo would have been able to acquire.
Well if they did, which I don't know about, they certainly would have known when sunrise and sunset would be, and accurately where they were in between, simply because they would have been used to using the sun as a way to tell the time. If you go to rural areas in England lots of people still exhibit the ability to do this. My own parents, who both grew up in very rural isolated areas, can tell the time accurately within 5 or 10 minutes without a clock at all.
If you have a chart, it's really not vague at all.
Hunting hours start 30 minutes before sunrise and end 30 minutes after sunset.