As others stated, there are many rough edges and footguns in the stdlib. BUT ... in my (and yours apparently) opinion, it's a matter of knowing those edges/guns, and work with them. Like you, I also prefer to create my own code around those instead of bringing in some library that brings in their own foot guns and possibly sub-dependencies and and and...
So your projects end up with their own "lib" of scattered time functions, possibly with new small bugs. I'd then rather have a proper well-tested and maintained library.
The "lib" one needs for date time functions is as good as non-existent.
But yeah, you will have to create tests for the codepath, instead of relying on the tests the library maintainers create.
Given the list of issues (most of which I had never heard of) that the library author describes with datetime, I think I trust his tests over the ones I’d write.
Given how many tests I see fail on 29th February at the companies I’ve been at, I don’t trust my colleagues’ tests either!
In a previous job where we had complete hell with an internal datetime lib under ambiguous ownership, a colleague joked that the original authors had taken early retirement when they turned 2,147,483,647 years old.