Where once there were many, now there are none. Time has done ticking; the hours are done. For now is an end, but also beginning, For darkest is now, but soon shall be thinning. What is it?
"Midnight" - Reasoning: The riddle is all about following something through to the end - a day. Once the day reaches its end, at midnight, when all the hours of the day are gone, right down to the last second, the clock resets. Thus, it marks both the end of a day and the beginning of a new one.