There is no notion of time in the brain because it is not absolute constraint of the environment.
There is no notion of time at the level of molecular biology, organs, systems, etc.
Outside of our minds, however, change does occur. Stars spend their fuel, galaxies spawn and migrate, rocks orbit stars in a periodic nature. The English word "time" is obviously a concept, and no matter which human word we use for "time", it's simply one way of measuring these changes we see around us. How can we measure the elapsing period over which we observe all natural phenomenon? That is the second, that is the unit of time.
There's also physics here: space-time. If we think of time as a concept of the mind purely, as you propose, then we also must consider space purely a concept as well, since Einstein proved space-time is a correct notion.
Specifically it comes from your SCN https://en.m.wikipedia.org/wiki/Suprachiasmatic_nucleus
The answer is it's evolved and some kind of complex cellular feedback mechanism, but Wikipedia is a bit light on details. Apparently this clock is also fanned out in some way to other parts of the brain/body which also follow a daily cycle.
https://www.sciencedirect.com/science/article/pii/B978044459...
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2735866/
Light isn't mandatory, but the SCN will synchronise to daily light patterns (eg when you travel). The SCN is also physically located close to the optic nerve. I think this is why the underlying rhythm is a slightly longer than a day, because it allows the body to reset it to 24 hours. If the period was shorter, even slightly less than 24 hours, maybe you'd have problems when the clock restarted before the day finished and the error would propagate over time (you'd keep shifting the cycle).
I've always wondered what would happen to the sleep cycles of astronauts on long space journeys. They may not have "daylight" to regulate their rhythms.
[1] https://news.harvard.edu/gazette/story/1999/07/human-biologi...
All definitions and measurements of time rely on counting events that are believed to happen at regular intervals. A second was defined for most of history as 1/246060 of a solar day, in other words, measuring changes in lighting.
We can tell time inaccurately by timing somewhat regular internal biological events. That pretty much follows from the definition of time as the interval between regular events (which are assumed for physical reasons to be equally spaced). The only question is which underlying events we use and how.
Explain apoptosis without change over time.
Explain radioactive decay without change over time.
Humans (and other living beings) can work with such low amount of waste heat and intake energy only thanks to atomically precise clocking.
At the same time, order of events is also a certain way of modelling something. There may be alternative models which can be meaningful without the order of time. But most (probably all) models are designed with the way we perceive time in mind.
Moreover, it is so stable exactly because it has neither the notion of time nor counting.
Cells literally have a clock pathway.
Any recommendations?
This one is about 'garbage collection' of cells: https://youtu.be/DR80Huxp4y8
And sequences of actions are maintained by pipelining of physical molecular structures - each enzime recognizes it's own unique patterns. That is it.
In old times this was called principle-guided science.
Understanding of principles spares one from knowing all the details.
Synchronization does not imply time. It implies synchrony, which is a mechanical notion. Electric signals are used everywhere in life for that purpose.
What they called clocks are not clocks. No more than a frequency generator is a clock.
Even if you're so pedantic that you're going to say something like "it could mean rate and not time". Rate also means "the number occurrences in a specific time interval". So how exactly are you contorting your logic to come to this conclusion?