Hello.. You did not specify the tech level of your civilization. If they are at XXI century level, atomic clocks can be an option. Other forms of time tracking are hourglasses. Or.. Well, maybe some cycles (no day/night cycle, ok, but maybe there is a moon orbiting, or at least they should get hungry or sleepy). Moreover they will have to move constantly (if they were on earth they should move about 10km each day at equator), so maybe they can measure it as the interval between two movings.

It's easier than when it's always day, but I don't see ultimately how the answer would be different.

Now I read that in another thread you wrote that there are a lot of moons orbiting the planet. So... Ok, measure time as moon's passes. "Oh, there it is Columbus, time to move again by 100km"

Here is a possible issue in this world: Your inhabitants are perpetually in an area that had been in darkness for 4.5 years (perpetual ice), or it had been in light for 4.5 years (perpetual drought). Your inhabitants have a choice of where they live, but the natural resources don't.

Do you mean the day is 9 earth years long or that one rotation occurs in the same amount of time as nine revolutions about its star?

@Octopus 9 earth years

@JoeFrambach True, but the different between the two isnt a sudden cut off, rather a fade; they live in the fade

Given the nomadic kind of existence, I'm not sure you would ever get to Midieval tech. Seems much more like Genghis Khan and the nomadic tribes. One question you should be asking is why do they need to know what time it is? Even Britain didn't care about anything other than "local" time until the trains got fast enough. And I'm guessing they don't stay awake for 4.5 years and hibernate for 4.5 years. What kind of crops could you have that lay dormant for that length of time as well?

soon or later they will build a railway,or move to the pole

It seems to me that time would be determined based in distance. I.E. Time doesn't matter. All that matters is that I travel 10km in what we would call a day. So if someone asked how long something took, I'd answer in distance. It takes 500m to wash the dishes. Maybe? Certainly would give some flavour to your storytelling.

Is the night side "certain death" simply because it gets so cold? In most places a certain depth below ground is always warm because the inner core redistributes the heat/cold equally, and also because the core of the planet might be molten. Could some people not find, and live in caves? Potentially sealing them off, except to enter exit.

@TrEs-2b Are you wanting this civilisation to count time or to keep a calendar? The two things are kind of separate things.

Related: What natural processes can cause a sidereal day to be significantly longer than a sidereal year?

Is it even possible to have a planet orbiting a star in such a way that one rotational period of the planet takes the same amount of time as nine orbital periods around the sun? I'm trying to wrap my mind around what that would involve in terms of simply geometry, and can't.

The sextant was invented in the 1700s. If you need it more, it'll get invented sooner. And your people need it more. All you need is fine metal working, and the ability to polish something to be mirrorlike. Bosh.

@MichaelKjörling Venus has a longer day than year.

@MichaelKjörling If our moon were the planet, and Earth was the sun, the moon would always have day on one side, night on the other.

♫ In daylights - in sunsets; In midnights - in cups of coffee; In inches - in miles; In laughter - in strife. ♫

@mbomb007 Well yes, because the Moon is tidally locked with Earth. Digging through all of the comments I now notice, buried among them, "Do you mean the day is 9 earth years long or that one rotation occurs in the same amount of time as nine revolutions about its star?" to which TrEs-2b responded "9 earth years" (which I take to mean that one rotation of the planet in question lasts approximately $9 \times 365.24 \times 86400$ seconds with an unspecified orbital period, but there has been no clarification in the question which is probably why I missed it originally.

Think about WHY they need to tell time first. Also, think about whether you might want your world to be one of the tidally locked TRAPPIST-1 planets.

well the romans used water clocks and candle clocks for day to day things. Plus they can still measure time by the rotation of the planet. They basically have had to since they know how fast they have to move to stay in dusk.

@MichaelKjörling Your assessment in correct. There is a series of linked questions by on the same topic by same author you can peruse, if you so desire. Here: worldbuilding.stackexchange.com/questions/43938/…

@Samuel Er, wut? A Venusian day is longer than a Venusian year. Only a little longer, but it is longer. So yeah, a point on the surface sees "about" half a year of daylight a day. Makes no difference to the fact that there is a real world example of a planet with longer days than years - which was Michael's question/comment.

@GrimmTheOpiner The sidereal day is longer, not the solar day, which was Michael's question/comment. This question is about solar days, is it not?

Are the inhabitants constantly on the move (e.g. on some kind of never stopping wagons / rafts etc.) or do they set up temporary camps and relocate them far enough from the twilight line every several Earth-days? (according to this rough estimate, on an Earth-sized planet, the twilight line travels around 12km (7.6 miles) per day - so both options seem possible)

No matter what physical solution you end up going with, as an aside, I kind of like the idea of a certain respected class of people who are official "time keepers", like some quiet organization of monks or something whose only job is to keep track of time and find ways to synchronize time across the planet.

@kingledion I shouldn't have to go dig through OP's other questions (only 362 of them as of now in this specific case) to maybe find details that are relevant to this question. Especially if a valid point is brought up in comments, the information should be incorporated into the question itself, and not be left buried in the comments. At a year and a half and over 27k rep, TrEs-2b has been here by far long enough that this should be second nature by now.