Normally when people say library they don't mean the built in libraries for a language but external packages.
Still, the title is overselling the post.
also, for the love of God, if you do any sort of date manipulation, just suck it up and use moment.
Moment is huge though.
I've used date-fns (https://date-fns.org/) with good success. The import structure allows for pretty good tree shaking. It's really good if you only want to format a date in like 2 places in a huge app - don't need to pull in the entire library for that.
I recently replaced moment (which is huge like others mentioned) with luxon (https://moment.github.io/luxon/). It has pretty much all the same functionality but a way smaller size.
Moment is huge, its performance sucks and the mutability makes developers shoot themselves in the foot constantly. Trust me there are much better alternatives.
For the love of God if you’re in the browser don’t use moment. It’s bulky and slow. Use date-fns and selectively import the functionality you’re actually using.