Marking time without light

I can imagine that after three days there would have been changed sufficient to determine the time of the darkness besides referring to the prophecies but I wonder about the three hours of destruction at the beginning of that darkness I wonder how they could determine with any accuracy (and without any mechanical record of time) how long that had lasted. In our day we could lose power at the beginning of the destruction and have it restored at the end and notice in some of the close the difference in the recorded time. Without any of that, how could sick a short time period be proven within the larger people of darkness. Not that their methods are important spiritually, just that wondering about such things makes them a but more personal and real.