# An immortal's measurement of time

A while ago I created a species of immortals that has existed since shortly (astronomically speaking) after the formation of the known universe. I've mostly ignored their society in the meantime as tangential to the primary story arcs they've appeared in, but this fortnightly challenge brought me back to them.

Our typical measurement is predicated on the movements of the Earth (days, years) and the Moon (months), and these values are incredibly short for an entity that can live long enough to watch stars form, age, and die.

There have been several questions around here about immortal entities, but I can't recall one that addressed how the entity in question would measure the passage of time.

So, how would an immortal entity born in the vastness of space measure time?

As there has been considerable interest in knowing a little more about my immortals, I've provided some of their details below:

• Able to manipulate space-time, including the ability to convert matter into energy and vice versa
• Originate from empty space, but built a planet to serve as their "home world"
• Capable of sexual reproduction
• Do not sleep, but do possess a rest cycle that accelerates healing
• Can consume material food, but can survive indefinitely on stellar energy
• Interact with mortal races on a regular basis
• Can view 90% of the light spectrum and limit vision as much as desired
• Can see into the future in near real time
• Immortal from the species' beginning
• @JoshuaHanley It's relevant, but I don't have a singular answer for it. We have eons, months, seconds, and even Plank time, so our measurement systems aren't always appropriate to every application. – Frostfyre Jun 22 '15 at 20:39
• Sexual reproduction actually doesn't make much sense in an immortal race; it's something evolving, competing organisms do. It may be a vestige of their biological past, but they'll have to exercise restraint if they don't want to cram the universe with their progeny. – Beta Jun 22 '15 at 22:11
• What does "90% of the light spectrum" mean? From what to what? Light spectrum can be said to be infinite. From highest energy gamma rays (highest measured energy is 16TeV, but there is a reason for energy above 50EeV and why not Planck energy or above) to lowest frequency radio waves (period 1/50 s is connected with standard European network electric current, but we can think even of periods of years and miliards of years) – BartekChom Jun 23 '15 at 7:41
• What does Able to manipulate space-time... mean? If it means what it says, then to them there are no fixed units of any of the four dimensions. They can stretch/contract dimensions essentially interchangeably (whatever manipulate means). What does 'measurement' of something you can change the... ummm... size(?) of mean? – user2338816 Jun 23 '15 at 7:43
• You should read up on the Q from star trek - they are exactly what you describe, and seeing how they are handled might help you. – Benubird Jun 23 '15 at 13:50

Measuring time is something we take for granted these days, but thinking back, it must have been a solution to a problem we faced long ago. We decided to create a measurement system that accurately predicts the sun's movements because we're a daylight reliant species who sleeps at night and grows food. Our planet has seasons which greatly affect our daily lives.

Now, imagine if we were to suddenly become immortal and we lived until well past the life of our original sun. How would that change our need to measure time? I'd imagine we'd keep the sun date system on some other planet (perhaps keep track of our original sun for posterity and phase in a second sun based time system). We'd likely mimic the familiar Earth time in any colony we make as well.

Therefore, the way to figure out how this immortal species tells time would be to ask practical questions and go from there.

1. Did they start out on a planet like Earth?
2. Do they sleep? If so, when?
3. Do they or have they ever relied on agriculture?
4. Do they or did they ever live on a planet with seasons?

If they don't sleep, eat food they grow or live on a planet with seasons, time can still be important, especially if they're a social species that doesn't have a hive mind (or is strongly telepathic). Regardless of whether they're immortal, they must operate with a frame of reference. That frame of reference can be temporary, especially if it's extraordinarily long lasting. If it is, they would likely have a system in place to choose another temporary frame of reference that is identical. They would choose something apparent and important, ideally.

That being said, I hear that white dwarf stars are incredibly long lasting. Their lives are said to be as long lasting as the universe. However, they're terribly dim and hard to see. I also don't recommend they replace a moderately sized star like our sun because it would be too cold, and the only planets close enough to have warmth similar to ours would be locked facing it, causing eternal day on one side and night on the other. Still, if one were available to view in the night sky far away enough that it doesn't lock the planet's rotation but also traveling in roughly the same direction and speed in the universe, it'd have potential to be a permanent frame of reference, especially if mapped from a rotating and spinning planet/colony ship. But I can't tell you how that would be practical.

I wish I could give you a more directed answer, but considering I know so little about this immortal race, it's the best I can do.

EDIT: Since I've obtained more context, I'm going to get more specific (while leaving the original answer intact).

Able to manipulate space-time, including the ability to convert matter into energy and vice versa

Considering they control space-time, it sounds to me like accurate time is both immensely important and not important at all depending on circumstance. Is this a technological advance? If so, then they'd likely solved the time measurement long before they learned how to manipulate time and perhaps revamped time measurement after the invention, and would therefore have two separate measurement systems (much like we have different measuring systems for temperature and size). The Gallifreyans of Doctor Who (also time-space manipulators with long lives) seem to have a universal time-line based between the big bang and the final collapse. They use important events as markers and have even developed timeline related senses.

Originate from empty space, but built a planet to serve as their "home world"

If not originating from a planet I cannot begin to guess at their physical size. (To be fair I can hardly guess their size if they had!) I'm supposing ants would measure time very differently from humans and if a galaxy were sentient it would measure time with a completely different (and impractical by human standards) system. I'll give the benefit of the doubt that these creatures are a size that can fit on the Earth and most other planets (AKA they're minuscule compared to a star). A point of reference would be what they base their time on, whether it be a migration, a star life or some other significant repeating event. I'm also going to assume they require energy of some sort, and therefore rely on stars, at least to a point. So some time system based on stars would make sense, even in absence of the context of a planet.

Capable of sexual reproduction

Sexual reproduction is often a sort of lifecycle event based in other natural cycles. What sort of lifecycle do these immortals have?

Do not sleep, but do possess a rest cycle that accelerates healing

The rest cycle is likely based on periods of inactivity and peace. Time measurement would take active events that interrupt inactivity and peace into account. The immortal beings would need to plan ahead in order to facilitate the rest cycles.

Can consume material food, but can survive indefinitely on stellar energy

They require stellar energy and would therefore find the lifecycle of stars extremely important. Their time would likely be based on that much like ours is based in seasons.

Interact with mortal races on a regular basis

If they interact then they would likely influence and be influenced by these mortal races. (Again, see Gallifreyans for comparison.) It is therefore possible that they can use the mortal time measurements for sake of the mortals. This is additionally feasible considering they are generally not the main focus of the story.

Can view 90% of the light spectrum and limit vision as much as desired

They seem heavily specialized in the light spectrum, which further illustrates that they'd likely base time on the lives and events of stars. Even if it were temporary, like "[Star Name] Era, [Lifecycle state]". When a star dies would be exceedingly important and noticeable.

Can see into the future in near real time

If this is an innate ability, they would likely measure time in events. Time's relative nature would be reflected in their measurement system. Our time is relative too; we have timezones and leap years and even have to adjust the clocks of satellites to fit our time. Time is relative. Time is not fixed. It does not contain an absolute, concrete and measurable attribute. Considering they're immortal and well aware of the transient nature of time, their time system might become more flexible than ours to account for it.

• Welcome to the site and good answer! I would think the abilities and history of my immortals to be mostly irrelevant to the main question of how an immortal measures time. I can, however, add some more detail if you think you can devise a more accurate/better solution. – Frostfyre Jun 22 '15 at 18:43
• Thank you. Time is relative in nature so I think the best way to answer your question is to come up with the problem before you come up with the answer in this case. I'd love to hear more details about your immortal race in particular; the context given might help to create a more practical solution not based in as much conjecture. – RPHolmes Jun 22 '15 at 18:52
• I've added a few species details to the question. – Frostfyre Jun 22 '15 at 19:09
• Thank you for letting me know. I hope my additional response helps. – RPHolmes Jun 22 '15 at 20:09
• No TL;DR answer? :( – Joze Jun 23 '15 at 12:28

Radioactive half lives are universal, i.e. they can be used independently of where in the universe these immortals are located. Different isotopes have different half lives, allowing for larger and smaller time units.

If one of our immortals wanted to refer to something that happened early in that being's existence, they might date the event "half of a bismuth-209 half life ago", which would mean about 10 billion years ago (source). If they want their friends to attend a housewarming party next month, they might use vanadium-48 and its half live of 15.9 days as a time unit to specify when the hors d'oeuvres will come out of the kitchen (source).

The universality of radioactive half lives would make for more practical time units than, let's say, the orbital period(s) of the "home worlds" the immortals created. On the other hand, there is a problem with this time measuring, and it is that it has to be a relatively "new" invention of the immortals, since heavy radioactive elements weren't formed during the Big Bang nucleosynthesis, but instead via nucleosynthesis in stars - supernovae in particular, and via interactions between energetic particles and stable isotopes (source).

One universal time unit thing that could have been constant throughout their entire existence (although it is entirely up to the OP to decide if that is the case or not), is the time it takes for one, two, or three (or however many many immortals are needed between the sheets) to reproduce themselves. For a human, this would mean a time unit of 38 weeks, counting from conception (source). It would be a time unit that can be used anywhere in the universe, and - time dilation due to relativistic speed effects excepted - would mean the same for everyone using the unit, no matter if they lived on a space habitat orbiting HDE 226868, on Io or planet Earth. I also suspect that this time unit will remain constant for a long time (although not forever, since natural and artificial evolution will likely change things for our species, including our gestation time).

There is also a possibility that they do not measure time at all. If they have no need for time measuring, they probably won't bother measuring it. The measuring of time is a solution to a problem, e.g. knowing when to sow and when to harvest, knowing when to hunt (day) and when to go back to the cave rest (night). The fact that your immortals are polyphasic sleepers (well, resters, to be honest) would probably also lead to blurriness of time. Yes, one could use resting periods and the time between them as time units, but what's to say that everyone will have the same schedule? If they don't need to follow a certain cycle (like day/night on Earth), they might all go on their different schedule and nap whenever they feel like it. Even in humans, so strongly bound to Earth and the Sun's rising/setting, it wouldn't be too far fetched to assume that polyphasic sleeping could disrupt the notion of time as most of us knows it, and giving the individuals a feeling of experiencing time much more as a continuous flow, than something that can be broken down into days, hours, minutes, etc (source). Now, the feeling that it would be chaotic or confusing to meet at a given time for your immortals without a precise time measuring convention, stems from our feeling that it would be inconvenient to hang around Betelguese for 55 000 years years, waiting for a friend to show up at Milky Way's best ice cream parlor.

A species uninterested or unable in measuring time is not as extreme as one might think when looking at it from our Western time-obsessed cultural viewpoint. There are actually several cultures on our planet that functions without units for time, one of them being the intriguing Pirahã people (source; source). Granted, there is a huge difference in living in a small community in the Amazon jungle without having to keep track or even think about time, and a another to organize a large, space faring society. But unless you, OP, can see very clear problems that would arise from not measuring the passage of time, it is perfectly possible for this species to be "beyond" time, in a sense. Which makes even more sense when contemplating that space-time is something relative, and your immortals have the ability to distort space-time themselves - and, consequently, to screw with any time measuring their peers may rely upon for knowing how many days of aging they have to wait until it is time to start sipping on the home brewed beer.

• Very detailed response and some good points. However, I have it on good authority that they aren't orbiting HDE 226868. :) – Frostfyre Jun 23 '15 at 2:23

I see one point here that may give a "native/base" way to measure time.

Capable of sexual reproduction

Do they have to procreate using sexual reproduction, or given their ability to manipulate space and time and construct planets, are they able to reproduce asexually? Are they capable of controlling the duration of their pregnancy?

Given these (rather baseless assumptions :) ) on the mechanisms they have for reproduction, could you not use the duration of pregnancy/incubation as a measure of time? You don't state how long a pregnancy is (or its variability), but since the giving of life could be considered to be a major event in most cultures, you could use this as a basis.

It has been 400 incubation cycles since that star exploded.

Or something like that :)

I'd say this question is more about the time-space relativity of the immortals and their physiology. Typically, scale matters, at least as far as us humans can tell. Tiny things happen very fast, such as atoms undergoing nuclear fission. Very large things happen very slowly, such as galaxies trapped in a black hole. So if they are truly immortal, they may be very, very large. Or have learned some tricks about space and time.

But what are immortals doing with all of their time? Are they massive wisps of energy, whose perception of time is so slow that rotating galaxies are akin to night and day? (If so, we'd have no chance of communicating with them, as for us to say a single word to them would take thousands of years.) Or are they more like us, doing things on a "daily basis", where a far more rapid measure of time is meaningful? Or perhaps they are a combination of both... able to sleep for millennia, but when awake, are able to interact on a time-frame similar to ours?

I'd bet that their units for time would include a lot more digits than we normally use. Their clocks might show a time of something like A:E:Y:M:D:H:M:S (Aeon, Epoch, Year, Month, Day, Hour, Min, Sec...), likely based on the creation of whichever universe they were in, or currently chose to be in.

Being "immortal" doesn't mean that time excludes them completely. If they sleep too long, they might miss something huge (like a supernova, another immortal species passing through, etc.) Which again begs the question, what are they doing with all that time? Are they alone in the universe? Do they have friends to watch a movie with? Eat popcorn? Go on a date? Or is it a solitary existence? Would a date last a day, or 1000 years in our time? Why not 1 day our time, so that they could meet 1000 partners in 1000 years?

I'm an engineer, and on the surface, it sounds fantastic to build things forever... but I know that eventually it would get boring and I'd quit. What happens when an immortal being has exhausted everything interesting to them? Do they go insane? Become evil? Lose all track of time? Commit "suicide?" Figure out how to be reborn? Create a world like Earth, and watch it destroy itself as the inhabitants battle each other over trying to decide if there is indeed any deity?

If their perception of time was always locked to ours, then they'd likely tire of everything pretty quickly. However if they existed outside of time itself, or with a non-linear respect (or dimension) to time, (or the ability to manipulate it) then that brings up some interesting possibilities.

• It's tangential to the question, but my immortals have the ability to manipulate space-time, have built two planets and a new layer in the multiverse, and amuse themselves by nudging the activities of the mortal races throughout the universe. – Frostfyre Jun 22 '15 at 18:40
• I've added a few species details to the question. – Frostfyre Jun 22 '15 at 19:11

Depends on their interactions with others and their memories.

Would they even care about time? If you are going to live forever unless you are 'killed' does the passage of time really matter? If they interact with mere mortals then they might need some kind of way to relate to them as far as time.

But If I lived for billions of years, I think counting the rotations of a galaxy might be a place to start. "Andromeda has made a half turn since the last time someone called me such a name!"

edt: If they interact 'frequently' with more mortal people then they are likely too have multiple time frames. The Galaxy clock for themselves, and more reasonably short time frames for the others they interact with. Quite likely adopting the other time units, making it easier to be understood.

• Are there factors that would have to be taken into consideration regarding the spin of a galaxy speeding up or slowing down? – Frostfyre Jun 22 '15 at 17:57
• @Frostfyre It's really the orbits of individual stars around the center, not the entire galaxy moving. – HDE 226868 Jun 22 '15 at 18:02
• @Frostfyre I don't think galaxy spin changes much over billions of years. Look at how consistent the earth's orbit is. A slight change of 5% in speed, is it really important to have exact number of 'years'. We are estimated to take about 1/4 of a billion (250,000,000) years to circle the Milky way. 20 rotations you are about 5,000,000,000 years old that's close enough I would think – bowlturner Jun 22 '15 at 18:10
• I've added a few species details to the question. – Frostfyre Jun 22 '15 at 19:11

Use variable stars. They change brightness in predictable intervals. Long period variables have cycles of up to a few years, which would be just fine for immortals. When a variable star dies, simply choose a new one.

Here's the light curve for Mira, one of the best-known long period variable stars:

Simply count the periods.

• I realize that the cycles here are short, but you can use multiple stars to transfer across the eras. – HDE 226868 Jun 22 '15 at 18:00
• If you hadn't used math/science for this, I would've given this answer with a much more fascinating point of reference: When the ancient Egyptians built the pyramids, their points all faced towards Draco, which was at the time the guiding star in the sky. A race of immortal beings could use a similar large-scale time keeping device to track time through the changing position of stars in the night sky. – Zibbobz Jun 22 '15 at 19:22

I personally don't think they'd care about time if they can 'manipulate space time' and are immortal, as you say. I wonder if they would really even be able to perceive it.

Because of their nature, and their origin in empty space, I don't think they would have a 'native' way to measure time's passage. However, you also said that they built a homeworld, and interact regularly with mortals.

I think the orbital period of their homeworld would be a good way to express time. Earth is one year, for instance. I would think since they custom made their homeworld, it's orbital period could have some significance.

Alternately, since time is not native to them, they might just translate time into whatever units the mortal they are talking to likes to use.

Finally, they might use something like the estimated lifespan of a star of the largest possible mass without immediately collapsing into a black hole. Large stars burn out quicker, so this would be something relatively easy for them to observe. You cannot use the smallest stars, because their lifespan is older than the current age of the universe (unless you're using a time cosmologically far in the future, not now).

Given they can manipulate space-time then they must be able to perceive proper time or they would be affected by time dilation effects.

Since they interact with mortals (presumable on a planet) then it would make sense that they adopt the choice of measurement of time the mortal species in question uses when interacting with them. This could be based on rotation of the planet, orbital period, etc.

In interactions with each other they will most likely have adopted a similar method of time measurement as we have ourselves, based on some fixed physical constant such as periods of radiation of a caesium atom at rest at 0K. However, this level of precision would be unnecessary in general use, after all we do not say “22 hours 31 minutes and 12 seconds ago I went to the shops”. They would colloquially use a more abstracted approximation such as the lifetime of small/medium/large stars which would not be precise but they would understand the approximate time scales being conveyed.

From the description that you have provided few things are assumed:

They are beyond our bounds of understanding!


Reason being is that they can consume energy and also they can manipulate energy into anything. What truly defines their lucid existence is the ability to manipulate time or that it should not come as a surprise given that they can manipulate energy as well.

They are creatures which can possibly be able to create humans or any other form of life. (Given that we can create A.I. and also that it can be said our consciousness is also energy that should they wish they can retrieve and re-create).

So all this leads to their measure of time being energy. In quantum physics it can be assumed that time and energy are trade-off against each other. Hence we can say that time-energy to be a single unit. So their unit of measure of time comes through their understanding of energy or rather since they can reproduce through medieval methods such as sexual intercourse it can be said their 'age' is denoted by the amount of energy that has taken them to grow up.

Ultimately we cannot classify their unit since we do not have such deep understanding of the working of matter and the universe. However since they have interaction with the human race they can easily convert their measure of energy-time to our understanding of time.

How much we an understand their existence and their measure of time would depend on how much you would constrain them.

• we can create I.A. What is (an) I.A.? – zovits supports GoFundMonica Jun 24 '15 at 11:04
• Thank you for pointing it out... it was supposed to be A.I. (Artificial Intelligence). – dxtime Jun 24 '15 at 17:01

It depends a little on their tendencies.

If they're artists and travelers anything goes and the movements of stars they attribute artistic or religious significance to might be their unit, if they have a set physical body then the number of fingers they have might influence their choice of units etc.

if they're a race of physicists and engineers I can imagine units based on "plank time" as there's a certain elegance to a time system based on the constants of the universe.

Plank time is a unit of time based on the time taken for light to travel the smallest possible unit of distance. This would also make some calculations based on time and distance more intuitive.

Do they have computers? binary has a lot of advantages for computing over larger bases. If they're highly involved with their computers and tech after that long I can imagine switching to a numbering system which links into that more easily, think base 2,4 or 16 rather than base 10 like us.

Can they move/think as fast as normal mortals? Faster? Slower?

16^36 plank time in seconds is about 1.202 seconds. (second equivalent)

16^38 plank time is about 307.8 seconds (I'll be back in 5 minutes)

16^39 plank time is about 1 hour 22 minutes 4 seconds (hour equivalent)

16^40 plank time is about 21 hours 53 minutes 10 seconds (day equivalent)

16^41 plank time is about 14 days 14 hours 11 minutes (fortnight?)

16^42 plank time is about 33.35 weeks (half year?)

16^43 plank time is about 10.23 years (decade equivalent)

I think this would have a certain elegance.

A lot of good answers based on time measurement, but I feel you're missing the point of what it would be like to be immortal.

I think they would have given up bothering to measure time at all. This would make them seem very different to what we expect. After all, why do something right now when it makes no difference if you do it later. And if you're immortal, later could mean tomorrow, or it could mean in a thousand years. After all, what's the rush!

In some form, people on Earth have always observed the passing of the sun across the sky, and have been telling time based off that in some way. After all, the sun has always been there reliably. (for us, anyway)

## Formally

Since Humans generally operate on the day-to-day timescale, and think ahead in terms of years, we have created many different scales between those measurements. But increase the timescale to more than a human's lifetime and I can only think of the terms "century" and "millennium" - millennium only goes 1000 years. (I know there are other terms, such as eon, but I'm guessing the majority of people hardly even know how many years that is) Do the immortals ever need to abstract many hundred-thousands of years into a new unit? If they do it often they will probably come up with a new word for it based off their more local timeframe that they use on their planet (like years) - or start with the larger timeframe and divide it into their more local timeframe. (Like days into hours/minutes)

## Informally

Your immortals are probably older than every other thing in the universe. Would it make sense to use their own lives as a time-scale? You could use their sleeping cycles, their eating habits, or maybe translate the length of their hair into the passing of time.

"Last I had seen you was.. must have been 15 [custom-unit] ago!"

In comparison, "Last I had seen you, you were this tall!" (use hands to indicate height)

Of course, consider that there is always some variance allowed in time-telling even in our society. Saying something happened "several hours" ago would, to me, be around 2-3 hours. Saying something happened "weeks ago" implies something that happened more than 2 weeks, but probably less than " a couple" months ago. This variance increases along with the scale you are talking about. For an immortals lifetime - this variance could amount to entire star's lifetimes depending on the timescale.

• This seems overly complicated. If one wanted to meet another and they weren't the same age, or a group wanted to meet, how would they know to meet at a certain time? I could see this getting confused a lot. – Frostfyre Jun 22 '15 at 18:15
• @Frostfyre For some reason I was thinking they were all around the same age and didn't create (immortal) children. They could still add their birth-time to the time that they have existed (as determined by whoever was "first"). I suppose a percentage time scale would be inappropriate for precise things such as meeting times, but the time for setting a meeting is local enough that it could be set by anything that is able to be known for all parties. (For instance, they could use years of some local planet, or schedule something at the death of a certain star) – DoubleDouble Jun 22 '15 at 18:42
• It's understandable. I excluded details about the species because I felt they were irrelevant to the main question, but it seems to be causing a small amount of angst for some answerers. – Frostfyre Jun 22 '15 at 18:45
• My answer has changed quite a bit - I'm attempting (somewhat unsuccessfully) to focus on the idea that they would create units of time dependent on what they most need to tell time for. People mostly deal with a daily timescale. If immortals are the same, they may just have a system similar to ours - possibly with more terms for longer periods of time if they use those units often. – DoubleDouble Jun 22 '15 at 20:13

Our time is based on the decay of Cesium atoms, not the rotation of Earth around it's axis.

Seconds are defined as a function of the half life of cesium, minutes and hours are defined in terms of numbers of seconds.

I suspect aliens would follow some similar arbitrary value for their base unit of time as a function of the decay of some radioactive material, and then would use prefixes similiar to our m, k, M, G, T etc to scale that up to convenient numbers to talk about very short or long things.

• Our modern second is based on the cesium-133 atom, but it wasn't always. – Frostfyre Jun 23 '15 at 2:26
• Nitpick: Seconds are not based on the decay of Caesium atoms, but Caesium-based atomic clocks observe electromagnetic transitions in the hyperfine structure of caesium-133 atoms and use it as a reference point. – DrCopyPaste Jun 23 '15 at 12:54

They could use as units the cycles of stable atoms in fixed temperatures, just like we humans started doing a short while ago. (we redefined the second in terms of it, and our fuzzier "day" and "year" units suffer adjustments semi-transparently to most people).

Another very good possibility is half-lifes of particular elements as pointed out by fantasia, but I disagree that they would use different elements for different timespans, I think one standard would emerge and they'd use multiples and submultiples of it (think "Giga" and "nano").

As for the epoch, i.e. the reference date, I see three possibilities:

1. The origin of the universe (our Big Bang, could be something else in your universe)
2. Some very relevant event in their history, e.g. the end of the Last Atomic War, the birth of Nul'Za'Thir the Bringer of Light, etc. We use the birth of Christ in most of the west.
3. An arbitrary choice that just got standardized, such as our Unix Epoch.

If these beings can live forever and see the future, it may be reasonable to assume the following:

1. They have excellent memory
2. They have a decent way to share knowledge/memories
3. They care more about what will happen than about when others will perceive this

As such I imagine the following:

• When talking amongst each other they focus on what, not on when
• If a when is required, they can refer to a what (before Alice was born and after Sun X went supernova)
• When interacting with other species, they can use the time system as it is used by those species, or relate to simple things (in 3 cycles of the moon)

Sidenote: This answer is inspired by the talk of senior citizens.

Keep in mind that time and dates don't have to use the same number system. Our counting system and time measurement use different bases thanks to the Sumerians: 10 base for counting and 6 base for time.

Also, humans neurologically perceive time as a function of space/distance. ex. Two years ago is a far away memory, two seconds ago is just behind you. Since your audience will be human, you might try a general sort of adaptation. What is considered a far away distance for them? Perhaps this is a year to us? Perhaps they measure every thing in relation to a science based system related to the speed of light?

However sexual reproduction doesn't make much sense for an immortal species. Why would an immortal species use a method of reproduction that relies on mutation and evolution as a foundation for progress?

Still, it would be interesting to accept this fact and see where it goes if you decided to give it a Douglas Adams type of sardonic spin.

For a true immortal that is able to manipulate space-time, traveling a million years into the past might take as much effort as it takes me to go downstairs to get some coffee (brb).

They could even see the past as another dimension, like looking across the room is for us.
Cat knocks a vase off and breaks it? Just reach over and grab it when it wasn't broken.

The past and future that we know could be a matter of distance more then a matter of time for them. The big bang is a few miles that way, and the big crunch is a few miles in the other direction.

The biggest question isn't how they would measure time (with a measuring tape of course) but how much work would it take to experience time in a second to second manner like we do, and what would we (or any time bound entities) look like to them?
My first guess is we'd look "flat" (sorta like what we would see if we were looking down on Flatland). Not 2 dimensional flat, but 3 dimensional flat, without that 4th dimension that makes things look like you could reach out and touch them...
My second guess is that we'd look like we were falling, unable to stop our decent through time toward out unavoidable deaths.

• ...to experience time in a second to second manner... Yep, it all very much depends on something like that. If their perception is such that they can perceive at nanosecond scale, things will seem very different from a millennium scale. But being able to ...manipulate space-time... makes it hard for us to figure any of it. If their perceptual 'second' was a millennium for us, I'd only exist for a small fraction of a 'second', making it tricky for them to 'interact' individually. – user2338816 Jun 24 '15 at 11:45
• @user2338816 I am with you right up to the If their perceptual 'second' was a millennium for us part. In this case, their perceptual second would have no fixed length in regard to us. A perceptual second could be a million years, or a nanosecond, depending on if they were moving or holding still in time. Maybe they could 'freefall' and experience time like we do... It's a little hard to wrap the mind around what it would be like experiencing time that way. – AndyD273 Jun 24 '15 at 14:19
• That's pretty much what I was getting at; but not very clearly. – user2338816 Jun 25 '15 at 13:26

They may use a concrete historical way of messure time instead of an abstract mathematical scale? As, it was before I destroyed Qweuxque but after they stole my vrålåk. Or officially, under the leadership of Gax.