time and the passage thereof
y'know, it's funny, really - most decades fairly fluidly run into one another, the borders between them becoming kind of their own little time periods of their own. the late '70s and the early '80s both had a very similar feel to them, the late '80s and early '90s had more in common with each other than they did with the rest of their respective decades, the late '90s and the (very) early '00s had their own neat little Y2K thing going on and the late '00s to the early '10s? barely noticeable.
meanwhile 2020 only started a little over three months ago and already the 2010s feel like a distant memory, holy fuck