Ask HN: What is some timeless knowledge for programmers / software developers?

I've been doing programming for almost 18 years and learned over a dozen languages and probably hundreds of APIs and libraries over the years. I no longer remember most of it mainly because most of it is replaced by something new. I am at a point where I feel resistance to learn a new framework or a library just because its new. Because i know where that new framework going to end up in a couple of years. I guess its a occupational hazard that our skills needs to be rewritten multiple times during our careers.

So i've been contemplating about learning things that last for decades. Something that does not go away out of fashion in few years. Math is definitely one of it. Once you learn Trigonometry, you are set for next few hundred years at least.

In terms of programming, what do you think is timeless knowledge and time spent learning it is a time well spent ?