There are good libraries and APIs for holidays, business days, working-day calculations, and timezone handling.
I built an API called Datewise for a narrower problem: when the answer has to be reproducible later.
Business-day and working-hours calculations depend on inputs that can change over time:
holiday rules, observed days, substitute holidays, weekends, and timezone data.
That means a stored calculation can become hard to audit later unless you also know which calendar dataset was used at the time.
