Since the introduction of the modern clock, telling the time is a non-issue. However, just imagine before the invention of the mechanical clock, when there were only rudimentary methods of telling the time.
In the past, one of the most popular methods of telling the time was by using a 'candle clock' that burned fairly reliably at a set pace past defined markers that indicated the time.
It was even possible to create a makeshift alarm by inserting a nail into the candle at the desired interval. When the wax around the nail melts, it would drop onto the metal plate below and make a noise.
There's no certainty over when candle clocks were first used, but they are no longer in use today, having being superseded by the mechanical clock. The earliest known reference to candle clocks was by You Jiangu in 520 CE.