Touche’
The Christian calendar was introduced in 525 AD. When it comes to referring to decades there’s very little need to pick one that works with the first one.
I did hear you correctly on this one. Your right zero does come before 1. And all actual decades start with a year ending with “0”. For example the 1990s, started in 1990 and not 1991.
Here’s a reference that might help:
I suggest you take a look at this reference I cited:
Well, maybe. Or maybe the 20th century just happens to lend itself to this sort of thing, in a completely coincidental sort of way, and now we’re out of the particular zone where each decade-or-thereabout has its own cultural identity.
There’s a lot of history before the most recent century, you know. Has the OP simply noticed that, with the 20th century, when you talk about the 20s. the 30s, the 40s, the 50s, the 60s and so forth, people will know what you mean, and the terms will invoke certain ideas and images? If so, then… well, good job, OP. I bet no one has ever noticed that before. Or is this “decadology” business supposed to have some kind of wider validity? If so, how do other centuries fare? Can you do this with the 1800s? The 1700s? The 800s? The first century? The 300s BC?
Heck, does it work in the 20th century in Britain? Australia? Germany? Soviet Union / Russia? Namibia? Why or why not?
These places at least have the benefit that detailed history, including living memory, is still available. Having the OP’s theories fall apart if applied to 300 years ago would likely be more due to lack of input data than to any inherent flaw in the theory itself.
Seems like (although I am far from an expert, and just, y’know, in my humble opinion…) you need a particular level of mass communication and development of new technology to keep pace with a “decadology” type of identification. Too slow in communication and technology development, and changes happen too slowly for 10 years to make a huge difference. Too fast, and everyone is onto the next thing at approximately the same time, and nothing lasts long enough to become a defining characteristic for several years. So I wouldn’t be surprised if the 20th century was in that sweet spot.
Certainly, in costumery, we can divide things into eras, but they’re generally longer than ten years for a long long time. While there were certainly differences between a woman’s dress in 1800 and one in 1830, you have to be pretty obsessive to notice. There was a big shift in 1835 or so when waistlines suddenly dropped from below the bust to the natural waistline, and longer skirts started needing petticoats to hold their shape. Then women’s fashion does become more and more identifiable with the decades - as the pace of technology in dressmaking fabrics and techniques and communication of those new styles took a leap forward.
Today, I’m starting to have trouble finding similarly rigid defining factors in fashion. There are so many options, from breezy bohemian flowy skirts to skinny jeans, that it’s hard to point to something and say, “Yes, this is the look of the 2010s.” Or maybe I’m just getting old enough to realize that bell bottoms and flares and bootcuts are all pretty much the same damn cut, so I don’t see as much of a difference between 1995 fashions and 2015 fashion as I did between 1975 fashions and 1985 fashions.
Sure, but we don’t discuss decades in the same terms that we do centuries. While I would say that I was bone in the 20th century, No one would say that I was born in the 6th decade. You can make a strong case that the sixth decade goes from '61 to '70, but you’d have to hold a very unique position to say that the Sixties don’t include 1960 but do include 1970. The very definition of the Sixties is the ten year time period inclusive of the years that contain ‘sixty’ in the name.
I see where the OP is trying to go though; there are definitely periods of time when culture/politics/etc… are different than the periods before and after. They also seem to align very loosely with calendar decades, so it’s a natural thing to conflate them and define them by calendar decades. The OP is trying to better define them is all, with the preludes/echos overlapping and the “zeitgeist” being the main thrust.
The problem is (as I was trying to get at earlier) is that often those periods are started or ended with a specific historical event- I’d argue that in the US, the cultural period that ran concurrently with the 1930s really ended at the end of 1941 with Pearl Harbor, just like the period called the 1990s ended with 9/11 and the post-9/11 tech crash. Those two events fundamentally changed things from the way they were in in the previous period.
Other periods are harder to define; the 1980s/1990s divide was subtle; if I had to pick a point when it changed, it was probably somewhere within 1992-1994 when Clinton was elected, grunge music became popular after the 1980s pop and hair bands, and the internet started becoming a real thing outside of academic computing circles.
But you’re right, the calendar decades are a sort of verbal shorthand, not a hard and fast rule.
It seems to me that we’re mostly speculating about where the OP is trying to go, since said OP seems to be rather coy, and very much into showing rather than telling. Personally, I would still like to have a couple of basic questions answered:
What, exactly, is “decadeology”?
Why is it such a contentious and inflammatory issue on other boards?
One thing I agree with the OP (the post, not necessarily the poster) on is that eras can overlap. I’ve found that as a rule of thumb, a year fits in to either the early or late part of a decade, and either the “turn of the” decade or the middle of the decade. So 1981 was part of the early 80s and also was part of the turn of the 80s, but 1984 was part of the early 80s but was also part of the mid-80s. 1979 was part of the turn of the 80s but also the late 70s.
Year cannot, however, in my opinion, share eras if they are more than 5 years apart from each other. For me that’s almost a firm limit. If I think about a song’s year and then think of another song 3 years away from it, they usually feel contemporary. 4 years apart and songs will usually seem more distant. Songs that are 5 years apart from each other will always seem out of place with each other to me.
This rule only goes for pop culture: 1947 was much more similar to 1962 in many ways than 2001was to 1986, but the music of 1947 versus the music of 1962 is not one of those similarities.
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It’s basically a way to discuss decades and even cultural eras as well. It would typically deal with a question like, “when did the 80s era begin?”, or “are the 2010s like the 2000?”.
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Other boards are so biased against it and they would close any thread dealing with decadeology.
What makes it an “-ology”? It seems to me that we ask questions like that around here all the time. Is there some methodology/science/mysticism/principles/whatever involved in your particular approach? Do invisible unicorns enter into it?
Why?
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That I’m not sure. “Decadeology” would be a sniglet, a word that should be in the dictionary but isn’t. I just basically heard of the term, and I doubt that it’s formal. And no it has nothing to do with unicorns.
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They just simply seem to be biased against decadeology.
I’d never heard of it until this thread. And so far you’re doing a darn fine job of convincing me to be against it as it’s merely an arbitrary label repeated ad nauseum by an obsessive. Who still won’t tell us anything beyond the name of the thing.
Pick your favorite decade, whichever that is. Or the one you’re most confident in your analysis about. Then explain your work. How and why do the changes between year A and next year B differ from those in next year C? When and why do these changes accumulate to the level of forming a new decade?
You’ve repeatedly told us the name. Cool. We get it.
Now turn it on so we can see it go. Please.
Things fluctuate. Often what appears to be periodic is just chaotic fluctuation on which approximate periods can be imposed.
In human culture, apparent trends may last for days, months, years, decades, centuries, or even millenia. Very brief trends get names (“the Access Hollywood Tape buzz”); very long trends get names (“the Mesolithic”, “the Bronze Age”). Some trends that, by happenstance, last 15-30 years get labeled as “the X Generation”; trends that, by happenstance, seem to last 6-14 years or so get labeled “the Y Decade.” Trends whose duration is roughly a decade or generation seem particularly pertinent in biography.
In adulthood, my own personal trajectory has been divorced from American culture, and I have very little idea what people are talking about when they mention “the 70s” or “the 80s.”
I did, however, have some formative years in the 1960s.
I think there might be 2 or 3 60’s Zeitgeists only loosely related. 
In 1966 we noticed a few long-haired guys at our high school hanging out together. “What was this?” By 1968, my own hair had grown out and even my short-haired friends were all smoking pot.
The Altamont concert was a clear symbol of the “Death of Hippie”; was it just a coincidence that it happened barely 3 weeks before the Odometer clicked from 196x to 197x? I happened to attend Altamont but for me it was a minor waystation between a bad 1969 and a bad 1970.
I see. Well, this is all beginning to sound very silly.
Here’s a tip, BTW: If you venture into CS and feel like discussing TV shows, don’t say that you’re doing “televisionology”. People might look at you funny.
Then how do you know this is how it should be done? Dammit man, I thought were were talking to a Professor of Decadeology from the Academy of Cultural Periodicity. Now you’re saying you just made this stuff up?
ETA: or is it “sniglet” you just heard, and you are a seasoned expert?
I suspect decadeology is what happens when a numerologist discovers a calendar or an old collection of Life magazines.
I’d like to see what happens when a numerologist into decadeology then gets into astrology. IOW, starts trying to incorporate those base-12 cycles into the calendric base-10 orthodoxy. We might get some truly yuuge head asplosions. Don’t cross the beams!! 
Astrologyology?