"Back in the day" is one of those odd little idioms. I suppose it is an abbreviated form of "back in the old days". More and more I notice that the "old days" are different for people at various ages. For example, I noticed a group of "born in the 70's" Facebook members. I immediately thought: Oh, young people, 38 at the oldest. Then I realized that "born in the 70's" was supposed to be a group of relatively old Facebook members. Most Facebook users seem to be born in the 80's or 90's. However, since I was born in 1950, I was calibrated very differently than most.
That is one of a number of things that lead me to consider how our time sense is calibrated. Like many folks, I have come to the conclusion that as we age, we tend to see a year or a decade as an increasingly small increment of time. Supposedly a 100 year old woman was asked how her days were going. She said "It feels like I eat breakfast every twenty minutes".
Getting more specific, I think we tend to compare any given length of time to the total time we are aware of having been alive. I have only vague memories from before I was about 6 or 7 years old. So I could use 50 years as my "conscious life". (I am 57). I find that something that happened 5 or even 10 years ago seems very recent. My twenty something children have a conscious life of 15 or 20 years. A decade is a much bigger deal to them.
This also seems to work for multiples of my lifetime. The year 1900 happened about two of my lifetimes ago. It seems astounding to me that there where no airplanes, no computers, no radios or TVs, etc. etc. Just two life times ago. Three life times ago it was still legal to own slaves in much of the United States. Four life times ago, there were a few primitive steam engines, but the vast majority of work was done with human or animal muscle. Oddly enough, the little bit of mechanical power that was tapped was all renewable energy, mostly sails and waterwheels. Wealthy men wore tight capri pants and powdered wigs.
Now consider multiples of 20 years. Two life times ago, hippies and Vietnam war protests were much in the news. There was no Internet, but personal computers were common. Three life times ago, baby boomers were being born. Formal racial segregation was rampant. TV was in its infancy. Four life times ago, the great depression was about to begin. Movies were still silent. Traveling on an airplane would be very rare indeed.
The time calibration also affects how we look at the future. It is unlikely that I will live another whole life time. For a twenty something, several more life times may loom ahead. If I live another couple of decades, I will find that I have at most a small fraction of a lifetime remaining to me.
Meanwhile, I find that one hour can still seem to creep by if I am doing something I don't like (waiting for an overdue airplane, or having a root canal done). One hour can fly by if I am enjoying a good book, good company, or a good meal. Every thing I know suggests that it is no different for someone much younger or much older than myself. So we all have twenty four hours a day for all the days of our lives. And each one of those hours may be agonizingly long or regrettably short. Yet when we look back or look forward, our sense of time is very much affected by the total number of those hours that we have lived.