Recently, I've invested time in learning the intricacies of Git and it has got me thinking about time and learning. (My previous experience with version control systems was only limited use of CVS and SVN.) It took me a whole day's worth of reading to be able to understand the concepts and differences of Git.

There are an infinite number of things available for us to learn. Some, more useful than others. I don't know Fortran - I'm relatively young. But looking back at the preceding years of my life, I notice that I'm busier and busier as time goes on. The amount of things I have to get through in a day is increasingly out of my control.

It doesn't take a genius to extrapolate that information and realise I'll have even less time in the future - unless I get fired, but I have no strong plans relating to that idea for now. So, given that I have much more time and energy now than I will have in the future: what have you learnt, that has a steep learning curve, that you would possibly recommend to a fellow programmer?

Edit: I've stumbled upon the excellent question What programming skills have provided you the best return on investment? and hav realised that my way of approaching how to spend learning time was naive - it doesn't matter if ten useful concepts can be learnt in the time of one if they're worth it.