The first result in Google [1] which I often use, lets me paste a unix timestamp in seconds, milliseconds or nano seconds, and it accordingly converts it to a Human readable string.
Is that not the feature you built this for?
It looks like it only gives millisecond resolution in the output, and only displays milliseconds if there is at least 1 millisecond. Give it 1677951565020123456 and then you will get 11:39:25.020.
How did this reach the front page?
You shouldn't view someone building a toy project in a negative light, even if it's for learning purposes. It demonstrates curiosity and interest in learning, and is part of what being a hacker is all about.
Maybe it's not frontpage worthy for this crowd, but it's certainly worth mentioning in a CV, especially for junior engineers. It can serve as a good topic of discussion during the interview, for example.