Being able to know the time at any given moment is a modern convenience. It wasn't too long ago that humans would pay to see the correct time, and it wasn't too long before that, given the length of time humanity has been around, that we were working out roughly what time it was from the shadow a rod left on a sundial.
Mechanical clocks, atomic clocks and shadows are not the only ways that humans have attempted to keep track of the hours. In fact, we've come up with a variety of novel methods, largely because sundials don't work too well in the middle of the night, or when it's a little too cloudy.
Water that tells the time
In Ancient Egypt, water clocks were used from at least 1350 BCE. These measure time by allowing water to flow out of a vessel at a known rate, while markings inside the vessel tell you what the time is. It sounds simple, but ancient builders had to figure out how to account for the flow slowing later on as water pressure dropped, which makes it a neat bit of engineering.
Even so, these outflow water clocks were largely used for measuring shorter periods, more like an egg timer than an actual clock. More complicated versions became available over time, including one that was reasonably accurate and used for a variety of purposes.
"During the first Alexandrian period, it was adapted as a way for physicians to count the pulse," Lisa Nocks explains in her history of technology. "It was also used in law courts to time speeches. A long tube was plunged into the water and when it was full, the opening at the top was closed. When it was reopened, the water dripped through a small opening at the lower end. A person was free to speak until the tube was empty."
Clocks you could smell
While these were good clocks, you had to use your eyes to see the time. In medieval China, inventors came up with a clock that addresses this, in the form of a fragrance or incense clock. These emerged some time before 520 CE.
The simplest of the burning clocks, not to diminish the thinking behind them, were candles that burned at a known rate, with markings down the side to indicate how many hours had passed since lighting. This form was the most widely distributed burning clock, but not the most sophisticated.
"There was the incense seal, consisting of a continuous trail of powdered incense marked off into equal periods to enable the observer to measure the passage of time by the progress of the burning incense with relative accuracy," Silvio Bedini explained in a 1963 paper.
But these clocks still required looking at things. In a considerable leap, clocks were developed that burned until they reached certain markers along their path, which then emitted their own smells. In certain versions, looking at which chimney the smoke came out of would also reveal how much time had passed.
"It is entirely possible that the pegs or tablets inserted along the incense trail to denote time intervals could in actuality be tablets of hard-paste incense, each made from a different recipe," Bedini added. "When the progressive burning of the path reached one of the markers, the marker too would be consumed, and the resulting variance in aroma would be detected by the attendant, so that he could tell the particular hour which had elapsed from the scent."
In short, you would be able to smell the time.
Some of these had an additional system of balls that would drop when the incense burned through, allowing you to hear the time change as the balls clattered into a tray below.
These clocks are no longer in use except as a novelty. The modern-day equivalent is identifying lunchtime by what a colleague is heating up in the office microwave.