Suppose a civilisation on a moonless planet that is in an orbit with so little inclination and so circular that there are no noticeable seasons anywhere on the planet. An alemna would be just a single point.
Would this civilisation develop the notion of a year?
If not, how would long-term timekeeping work? "750,000 days CE"?
Yes.
There are many cultures on earth that don't really experience a change of seasons, like most of the south pacific islands which only really experience a 5 degree temperature change between their hot and cool seasons ("summer" and "winter"). A world that experienced no change in seasons at all would still find something to measure time by.
We base our years off of one complete orbit of the earth around the sun, but cultures like the Maya (who only experienced two seasons: the dry and wet seasons) had several different calendars, including a 260-day calendar, with 20 periods of 13 days used to determine the time of religious and ceremonial events. Each day was numbered from one to thirteen, and then repeated.
Every culture measured time differently, but in most cultures the calendar was based off of watching the sky. Most people in the western world have never seen the night sky due to light or air pollution, but back in the day everyone stared at the heavens in wonder and amazement, many mapped the stars and their movements, navigated by them, and looked up into them for understanding. Even a planet with an exceptionally plain orbit would still have a great view of the cosmos and passing asteroids. They perhaps could measure years by the annual appearance of an observable planet or comet that passes near their orbit every few hundred days or so.
As long as there were days, and their planet moved through space, there would still be some meathod of tracking time.
They would know about years, but it likely would not be the most common tracking method..
They'd be able to determine the length of a year fairly early on using astronomical observations of other planets. This could happen fairly early, but would be more of a scientific curiosity than something that would impact daily life. If there are no other planets (unlikely, but possible) then they'd need pretty advanced telescopes and scientific knowledge to figure it out.
For common use there would probably be some sort of artificial time construct, measured in days, that was used for bureaucracy and bookkeeping. For example they might track somewhere in the 30-1,000 day range, and use that in place of our "year".
With no moon, no inclination, and no seasonality, there's no reason to distinguish planetary rotation about the sun (although an elliptical orbit would still produce seasons, albeit almost unnoticeable...)
They might make "weeks" based off of digits-either five for one hand, ten for both hands, or twenty for fingers and toes. Finger/toe counting of days is always accessible, and might form their "month," but there are more useful measures.
In all likelihood, they would measure time by crop plantings (I was born thirty harvests ago) or some regular weather event. That would easily happen once they reach some agricultural milestones. From there, they begin to abstract: I will pay you in two harvest's time, this house costs two bushels every harvest cycle, etc. So "months" become some predictable harvest cycle, perhaps for grain
"Years" might be a longer growing plant's harvest cycle, such as nuts, which is expressed as a multiple of months: "There are ten wheat harvests for each nut harvest." Or it would be the time it takes for a tree to viably produce its first harvest. This would make it an important economic marker: "I will pay you back when I have my orchard grown" is expressed as "I'll pay in one tree."
So someone would say: "I am thirty nuts/trees and five wheat old."
