> Taking on a dependency does not relieve you of reading docs, but it also adds a further burden as you now need to trust the code. The stdlib is much more heavily tested and documented than any 3rd party library will be.

Sure, but the opposite applies as well. Sticking with the flawed stdlib means you are trusting that every single future developer is as careful in reading all the docs as you are - even when it's someone reviewing that random trivial-looking patch which is secretly hiding a massive footgun. A junior developer submitted a five-line patch touching datetime? Better schedule several hours for a proper analysis!

Or you can write your own wrapper code, of course. Which will almost certainly have worse documentation and testing than a popular third-party library.

External libraries aren't evil. When chosen properly, they relieve you of burdens. You shouldn't grab any random "leftpad"-like dependency like they are going out of fashion, but something as complicated as timezone handling is best left to code written by domain experts - which means using a library.