Yesterday, my parents had been dead for 45 years. Every year on that day, I subtract 1965 from the current year and come up with how many years it has been since that day. This year, I came up with 45. Forty-five is a nice, roundish number, midway between perfectly round numbers. It's not as round as fifty, midway between zero and the century mark, but it still has a "plunky" feel to it, as if some milestone had been reached.
Studies have shown that merchandise sells better if the price ends in "99." For example, $5.99 is supposed to sell better than $5.82, even though $5.99 is more money. It's all because of that round number feeling, and how round edges are easier to maneuver around than straight, square edges. Also, though $5.99 is only a penny away from $6.00, it still sounds like it's $5.00 and some change. But really, you may as well mean $6.00. That's what people go for though. They go for edges that are easy to maneuver around.
Yesterday, I printed photos of my grandchildren and tacked them up on our new display wall. So far, the grandchildren have the idea that you always get as much good food as you like, and all the fresh water and juice you care for. You're never too cold or too hot. There are plenty of things to play with and all kinds of things to explore. When you get sleepy, you just go to sleep. When you're done sleeping, well, it's pretty much time to do whatever you'd like again.
I am sure that someday they are each going to realize that there are other parts to life that aren't as welcome, and they may also subtract one date from the current year and reach milestones. But for now, all they have to do is whatever they feel like doing at the moment. They don't even know how to subtract yet.