In regards to fantasy dates, they're usually set somewhere between 100 - 9000 W.E (Whatever), but rarely exceed 10,000. Is there a reason for this besides being more aesthetically pleasing?
More importantly, what happens when the date becomes too long? It seems counter productive to be writing the year as '300,000,000 BC/AD" So what would happen if it ever came to that?
Fantasy and real world dates rarely pass 10.000 because cultures don't tend to survive that long. Date systems generally start with a significant event as 0: rise of a king, birth of a prophet, founding of an empire, solar eclipse etc.
Between those moments and the fall is rarely more then a thousand years, two thousand perhaps. I figure that if the system becames too broken we would just adopt a new one. The gregorian calendar wasn't the first one we used.
Only recently we have been even contemplating time periods longer than thousands of years ago. Having a need for such a system implies an advanced civilization. One who undoubtedly knows many date keeping systems. If we ever spread across the stars a central date keeping system becomes pretty meaningless too. Days and months vary per planet, traveling near lightspeed causes all sorts of weird effects on aging etc.
Take a look at how real-world calendars have dealt with this.
The infamous Mayan calendar, for example, counted days from a start point in 3114 BCE, using 5 periods. Each period counted from 0 to 19 then rolled over back to 0 (except the second-shorted period, which had a period of 18). Thus each period represented a set number of days.
August 12th, 3114 BCE was 0.0.0.0.1.
August 13th, 3114 BCE was 0.0.0.0.2.
September 1, 3114 BCE was 0.0.0.1.1.
January 1st, 1 CE was 7.17.18.13.1.
May 22nd, 2017 CE is 13.0.4.8.13.
Using this system the Maya could track dates from the year 3114 BCE up to 4772 CE with ease. You can use something similar to track large expanses of time in your world, especially if you're willing to use periods greater than 20.
For example, we westerners might like to use a structure of Year.Month.Week.Day, especially if we were willing to make our months a little more sensible and set them all to 28 days long (1 lunar month). This year would begin at 2017.01.01.01. We don't generally use 0s in our time counts, so we're starting at 1 for each period. Today's date (as I type this) would be 2017.05.01.01
To make this truly useful to track very large dates, though, we should add a fifth period. I would suggest the Great Year - the time it takes Earth's axis to precess a full 360 degrees. A Great Year lasts about 25,800 years; we could divide that up into 12 periods to represent the signs of the zodiac, giving us a period of 2,150 years per constellation.
Today's date 01.2017.05.01.01
January 1st, 2200 01.50.01.01.01
January 1st, 10191 04.1591.01.01.01
If you want to go beyond the year 25,800, you could add yet another counter, to track the Great Years:
January 1st, 426,255: 16.06.555.01.01.01
For common use, of course, you wouldn't need the first two periods - the current year and date would suffice. The full count would only be used in formal settings, similar to how legal documents often spell out the year in full (Nineteen hundred and whatever).
It gets a little convoluted, with all the different values being used, but it can track some pretty darn high dates with great precision.
Also, there is an excellent chance that I've fouled up the numbers somewhere up there - I'm a logical guy, but dyscalculia is a thing. Feel free to edit and correct any numbers I've screwed up.
This is an interesting problem to tackle because it deals with timescales that most of us are very unfamiliar with. My first recommendation to anyone dealing with such timescales is to read Stephen Baxter's book Manifold Time. It's one of the few books I've have seen which truly grasps just how strange life can get when making sense of how things are in the year 1,000,000,000,000,000,000,000,000,000,000,000,000,000 AD. Yes, that makes 300,000,000 BC/AD look pretty tame by comparison, and yes, Baxter goes much further into the future than that!
There's a few solutions to the issue you describe which have some history. The first key to all of these is that we are really trying to describe a "time point," and we typically want to do so using a number. The second key to this is that the purpose of such dates is communication. Our goal is to convey the concept of the time point to another individual.
Cycles are a very popular approach to dealing with large dates. Instead of counting numbers of days, we count in months, then years. Other cultures, such as the Mayans, had even more exotic cycles.
The fundamental reason cycles work is because the events we care about tend to occur on logarithmic timescales. Consider how many things we care about happening in a day. You might have 5-10 points in the day that matter enough to pay attention to. Now think about a month. Obviously you don't pay that much attention to the 150-300 points in the month corresponding to the 5-10 points every day. That'd drive you bonkers. Instead, you have 5-10 really important things to look forward to in the month, like pay day or someone's birthday. Look at a year. You might have 5-10 major events that you truly care about, like a family vacation or a major release of a product at work.
We currently are living 2000 years after the epoch of our current numbering system for years. This can be inconvenient, so we are known to shorten it to a 2 character year for convenience. As long as we're certain that '05 means 2005 and not 1905, this approach is very effective. Most time points we want to convey are within one lifetime, so we only need 2 digits.
If you think about it, this is really just a special case of the cycles solution above, which only works on cycles of 10 or 100 years (or whatever based numbers your civilization uses).
We're known to invent new epochs from time to time. When we do this, we define a new time point to base everything off of, and do math to convert one time into another. The most famous of these right now is the Unix Timestamp, which counts the number of seconds since Thursday, 1 January 1970 UTC (sans leap seconds). Why was 1970 chosen? Well, there's many reasons but one of the major ones is exactly the issue you describe in the question. If you're counting a number of seconds, that number can get big in a hurry. 32-bit computers could only count to just over 4 billion seconds, so they had to choose a time which meant most times that people in the computer age could care about fit into that window. 1970 fit the bill. Of course, they're running into trouble soon: on 19 January, 2038 03:14:08 GMT, the Unix timestamp will run out!
There is often a meaningful time where an epoch gets specified. Historically, many dates where given in terms of the ruler's reign. You might talk about an event that happened 5 years into the rule of King Henry. In more modern times, we defined a very important epoch for humanity in 1972, when we defined UTC in terms of atomic clocks. Nobody had every defined the second so precisely, so there was no way to determine exactly what the current time was when setting the first atomic clocks. The solution was that we defined a new epoch, such that all clocks would read 0 at that time.
Interestingly enough, we've also used the act of defining new epochs to go the other way. The single most common way dates are measured in the high-precision community is as a Julian date. For example, a major point in timekeeping forked off 2 additional time systems on Julian Date 2443144.5003725 in order to deal with time dilatation effects that had been unexpected until that point. Note that that date number is pretty big. Julian Dates are based off of the number of days since noon on January 1, 4713 BC, on the proleptic Julian calendar. That odd year number, 4713 BC, was chosen intentionally to be far in the past. It was designed to be so far in the past as to occur before any recorded history (as well as being the conjunction of several cycles deemed important at the time).
This is a bit tricky, because it doesn't fit well with the math, but we measure time with respect to the present all the time. We talk about something that happend 3 hours ago, or 4 days ago. This approach has a disadvantage of being only valid at the moment in time where the date was uttered, but sometimes that's enough. We're used to working with a web of relative times.
If brute force doesn't work, you're not using enough. Humans may have trouble with large numbers, but computers don't. If your civilization becomes highly computerized large timestamps become easy to handle.
NTP timestamps are an excellent example. Right now they are 64 bit structures: a 32 bit integer number of seconds which will roll over in 2038 (with the Unix timestamp), and a 32 bit integer number for fractions of a second. However, there is talk of making them 128 bits long. This would be sufficient to measure times as "small as the time it takes for photon to pass an electron," and "large enough to be valid until the universe goes dim."
The key to this is that computers can handle these numbers so well. To a computer, the year 2015 is no more difficult to capture than Julian Date 2457023.5. Our number systems, with its place-based notation grows logarithmically, so each additional digit gives us another 10 fold increase in numbers.
This works great until the energy cost of storing a 128-bit timestamp in memory starts to become important. But for that story, read Steven Baxter's book!
It's worth noting that we don't even use full dates (100% of the time) in modern times. Most of the time it's simply 96, 01, 17, etc.
To answer your question, official records could contain the whole year while letters and casual reference would likely refer only to the last two digits. Humans generally don't live beyond double digits, so only referring to the last two digits allows others to understand what is meant.
Alternatively, the solution could be to add letters.
Starting at 10,000 it would be A0000, and the last usable date with that system would be Z9999. This would cover the span of 260,000 years.
Let's assume that science have taken over the world. And they start counting time from the birth of our Sun. So they go with 4,5 bln. But hey, it's ok if you tell time in the billion years event, but when you go to millions you go 4 mln. And thousands? you go 4k. and hundreds? just write 86.
There are events that can't be put in the month/year/century/ bracket. For example the time when dinosaurs walked the earth. You just can't pinpoint "from Monday about 5 o'clock till 5 millions years later Friday around breakfast".
Look at different example. When somebody ask you for time do you answer "first day of the fourth week of the middle month of the second quarter of the seventh year second decade of third millennium" or you just go "4.35"?
After sometime you just stop counting because YOU DO NOT CARE. Even if you are a deathless Emperor on a golden throne you just go with 40K because you don't care about days or months.
There are a few precedents for solutions to this sort of thing.
The Elder Scrolls series of games uses Eras in its timeline, while Tolkien's Middle Earth uses Ages in its timeline
In both of them, these eras are divided not by a set amount of time but by significant events. In the Elder Scrolls mythology, a few are started by someone founding a dynasty or empire, or even by declaration. Some eras are really long, others are much shorter. With Tolkien, the start of specific Ages seems less clear.
Regardless, this method allows for ease of remembering how long ago something is while not necessarily bogging things down in super-specific definitions. If your story is in the Fifth Era, and you reference something in the First Era, you can tell - without knowing how long any of the eras are - that it was a Long Time Ago.
The Human Era or Holocene Era dates events from an arbitrary date of 10,000 BC so that no historic or recent prehistoric dates have to be counted backward. Thus the current year is 12,017 HE as well as 2017 AD.
The advocates of that calendar see no problems with dates over 10,000.
Astronomers use the Julian Day number, counting the days since January 1, 4713 BC. The current Julian Day number when I write this is 2457893. That is 2,457,893.
According to this the value of the Vulcan Old Date in some Star Trek fiction can equal or exceed 140,005. This may be in Vulcan years or other Vulcan time units.
http://wrstone.sdf.org/startrek-timeline/notes-12.html1
Possibly people can use two or more different year counts. Perhaps proclamations and rewards for criminals can give the date both as year 25 of the reign of Emperor Alynomern CCXXXV, 10,001st emperor of the Atlantean Empire, and also year 270,025 since the founding of the Empire. Young people may tend to use the regnal years of the current emperor when calculating their ages, while older people have to use the years since the founding of the empire to calculate their ages.
And maybe some people also say the year is 186, counting since the latest war with Lemuria. And perhaps to some people the year is 5,751, counting since the foundation of the dynasty of the present emperor (Alynomern CCXXXV) which is the latest of the recent short lasting dynasties.
And maybe in the introduction you can say that the story begins late in the history of Atlantis, "only" 154,825 years before Atlantis sank under the sea! That will certainly give you enough time for Atlantean sequels.
The story "Out of the Aeons" by H.P.Lovecraft and Hazel Heald, concerns a legend of the lost continent of Mu that is calculated to have happened similarly far back in the past.
It was in the Year of the Red Moon (estimated as B.C. 173,148 by von Junzt) that a human being first dared to breathe defiance against Ghatanothoa and its nameless menace.
With computers, the date will never be too long.
So what if the year is 654,546,534,120,296,145? A computer can easily store and communicate numbers like that. And that far into the future, we could have computers implanted in our brains so we can do complicated arithmetic in our heads, remember everything and communicate as quickly as a wireless data transfer. Whatever problems exist in that year won't include the size of the number.
What date system would be used
If the years got to giant numbers (like the year 3,000,002,017), then a system could be used that writes the last few dates, then specifies how many digits are excluded.
How the date would work
The date would look like: 8x:17. the 8x would stand for the number of digits in front of the 17 and the :17 are the last digits of the date. This system would be highly versatile as the precision of the date could be specified by the user, for example, a precise, important date could look like this: 0x:3,000,002,017 (or just :3,000,002,017 would work fine) but a quick casual date would look like this: 9x:7.
How the date would be said
For the date 8x:17, the date could be said formally like: "Eight times seventeen", but in casual speak, the date could be said like: "Eight, seventeen" and it would get the point across well enough.
