“Never roll your own crypto” is just this year’s “never roll your own date library”. There will always be something. Could I code this? Even if it’s quick, there’s ongoing maintenance cost and you lose out on the FOSS community identifying and fixing vulnerabilities as well as new features that you may need to use. Yes, the library might be large and contain things you don’t need, but that’s the tradeoff. You can mitigate this (depending on the platform and language)—for example, with ESM tree-shaking.

I’d rather install date-fns or moment and let it decide what the fourth Sunday of a given month is in 2046, and also audit for the latest browser attack vectors.