Having found the answer on the Apple Development forum unhelpful, I did a global web search and found out the following:
To discover the release of Swift that one is running, you open Terminal and type "xcrun swift -version". From that, I find that the version of Swift I'm running is 3.0.2.
Now: the highest release of Xcode that runs on El Cap 10.11.6 is 8.2.1.
The highest release of Swift that runs on Xcode 8.2.1 is 3.0.2.
The doc for NMDatePicker requires Swift 5.0 or later and El Cap 10.11 or later, so it won't work on my system.
Well, why don't I upgrade? I will eventually, but when I migrate to a new release, I follow these steps:
1) make a point-in-time (PIT) bootable backup of my current system using SuperDuper on an external drive
2) perform a clean install of the new OS on the hard-disk of my MAC
3) make a PIT bootable backup of the clean install on the external drive
4) restore the PIT backup of my original system back to the hard disk of my MAC
5) boot the clean install off of the external hard drive when I want to work on it
Of course, if it's a ".1" version of the OS, I don't upgrade. This is the only way I've found to move to a new release safely and without issues, in that I now have my original known working system back on the hard disk of my MAC, and I can work on resolving bugs, problems, INOPs or whatever on the new system at my leisure. BTW, this is the same procedure I follow when I do Windows upgrades. Burnt once, twice shy.
As you can see, the upgrade process I follow is non-trivial, so I don't upgrade unless I need to, especially with Apple releasing new versions of its OS every 6 months or so. Of course, if I'm going to continue learning to do development on the MAC, I guess I need to upgrade now.
Anyhow, thanks for listening.