Time is a tricky thing. It can slip through our fingers like sand, or it can sit immovable as stone, as humans we have trouble measuring time objectively, and so through ought history we have found many ways to measure such time. How a civilization measures time can be as telling and interesting about them as any religion or weapon.
As with our previous article let's start by clarifying why we break time down the way we do.
Sixty seconds in a minute, sixty Minutes in an hour, 24 hours in a day (12 of day and 12 of night). It's a clean simple system, but why is it our system? It's widely believed to come from ancient Egyptians, who used a base 12 method of counting by tracking the joints on each finger. I am not however by any means a historian. But the key point is that the foundation of our measurement if time is relatively meaningless in the grand scheme of things. That being said, the vast vast majority of roleplaying games carefully divvy up their turns and actions and effect durations by measuring these times, so changing them must be done with an awareness of how it influenbces the rules. So let's start by looking at how they can be tracked, and what those methods say about your world.
Real world time methods:
Water clock: A water clock is a deceptively simple device and there are many concepts and designs. The most basic of which is a simple bowl with a tiny hole in the bottom set in a larger bowl. One form has the water in the lower bowl and the tiny hole slowly steadily allows the top bowl to fill up and once it sinks to the bottom a full hour has passed. Another variation has the water drip from the top bowl into a lower bowl until the top one is empty. Obviously these are not the most accurate methods of time measurement, and in order to keep track of a full day there must be at least two people trading shifts perpetually watching the clock and marking the hours.
Incense clock: An incense clock is a brilliant invention that uses the burning of incense to measure the passage of time. This can be done several ways. Some methods attach weights to a stick of incense and when they burn the weights fall and strike gongs or bells. Other variations can measure the reduced weight of the incense holder as it's bounty burns causing the holder to raise along a measured line. Another potential variant could capture the smoke and the increased weight of a smoke filled chamber to active devices. The advantage of these clocks is that they only require maintenance when they are done measuring, they smell fantastic and can alert many people with the ringing of it's going, and depending on the calibration they can measure minutes hours or even days.
Candle clock: The candle clock is another simple concept, it is essentially a candle which burns at a steady measurable rate. The rate of burning is used to mark the passage of time usually by measuring the height of the candle but there are many variations. One method features holes in the base of the candle which allow wax to slowly drain out and this trigger a weighted mechanism which will lift the candle to different heights, and a fantasy extravagant method could even use this movement to light the next candle in a long sequence. Another great example is mixing certain metals or powders into the wick, causing the fire to either pop with considerable noise, signaling chimes of the hour, possible even multiple pops for different times, or it could cause the candle to burn with different colours for different times. These types of clocks must be carefully crafted to ensure the candles are always the same consistency and size, and once they have finished burning they must of course be cleaned and replaced.
Sundial: The sundial is a classic time telling method that uses the rotation of the earth around the sun to cast a shadow around a circular marker that denotes the hours. While there is a great deal of science involved in how the sundial works they have been used since ancient times, outdating many of the clocks on this list. The advantage of a sundial is of course that if it is properly calibrated it remains accurate for an incredibly long period of time. The downside of course is that it doesn't function without the sun. And of course, if your setting has different days or years or a non 1 number of suns the sundial might not be very useful.
Hourglass: The hourglass is a beautiful and ornate method of timekeeping. Great blown glass bubbles filled with sand mounted usually on a wooden frame. The sand takes a specific amount of time to pass from one bubble to the next and then the glass is inverted to start all over again. To keep time for a large area you would simply have someone watch the device and then bang on a gong to alert a those nearby. The downsides to this device is that it does need to be reset and flipped back over regularly and that the art of precision glassblowing is a difficult one, making these devices hard to mass produce.
Mechanical clock: Of course the mechanical clock is also possible in many fantasy settings where clockwork golems and machines can wander the earth or slumber awaiting a target. Clocks are reliable, generally portable, can include mechanisms to set of bells or tones and need little maintenance. The downside is of course how intricate and complicated they are, which general leads to them being quite expensive.
Other time keeping methods:
Log: The time it takes a log of wood to burn. This is very roughly used as a replacement to an hour. “You sleep for 4 logs, then I sleep for 4 logs”
Wander: The time it takes a child to grow bored with an activity and wander off. A very informal measurement of time but usually is about a half hour to an hour. “I saw the ogre I did, wasn’t more than a wander ago”
Stick: The time it takes a twig or branch or stick to burn, usually equates to around a minute but of course far less accurate.
Arcane measurement: This one might get a little bit meta but one could also measure the passage of time by the duration of spells. Many spells have very specific durations, and cantrips are cast able any number of times. someone measuring a minute need only cast a dancing lights spell for a noticeable coloured glow that will fade after 1 minute. Likewise an object created by prestidigitation will last 1 hour before vanishing.
Lifespan: In a fantasy setting with all sorts of amazing creatures it would not be hard to imagine a creature with a very specific lifespan. Imagine a beetle that, barring unnatural circumstance, would live exactly an hour before dying and becoming a larvae with an hour long lifecycle before becoming a chrysalis for an hour and then sprouting into a beetle again. Such creatures would be kept and bred in order to tell time more effectively, doubly so if all of them in the world somehow operated on the exact same cycle, perpetually in the same state as each other.
Phenomenon: Perhaps your world has a reliable natural phenomenon. Perhaps the sky changes colours dramatically and noticeably at certain intervals. Maybe the tides of a nearby ocean are so even and perfect that one could set their watch by them, or you could have geysers or tremors equally reliable.
No time: It's also considerable that there may be no way to reliably tell time. In a place like D&D's abyss, or in a real or roiling chaos, or a primordial proto-dimension there may be nothing consistent and reliable enough to tell time by. Creatures living in such a place may be dramatically warped and different and may not be able to easily grasp the concept of time even if it were explained to them.
Other: Your setting may also lend itself well to any number of other unusual or fascinating time keeping methods, and I strongly encourage you to consider carefully the timekeeping methods unique to your world.