Date-fns and ms serve distinctly different purposes in the JavaScript ecosystem. Date-fns is a comprehensive date manipulation library offering 210+ functions for parsing, formatting, calculating, and localizing dates, built around a functional programming paradigm with tree-shakable imports. Ms, on the other hand, is a minimalist utility focused exclusively on converting between millisecond values and human-readable time strings like '2d' or '5h'—nothing more, nothing less.
This comparison matters because developers often face confusion about which tool to reach for when dealing with time-related tasks. Date-fns targets applications requiring robust date handling—calendars, schedulers, reporting dashboards, or any system with complex temporal logic. Ms appeals to developers who need simple time unit conversions for timeouts, cache TTLs, rate limiting, or logging without the overhead of a full date library. Understanding their distinct strengths prevents over-engineering simple problems or under-tooling complex ones.
Choose date-fns when your application performs any meaningful date manipulation—formatting user-facing timestamps, calculating date ranges, handling timezones, or building calendar features. Its tree-shakable architecture means you only pay for what you use, and its performance with native Date objects makes it the best choice for computation-heavy scenarios. The learning curve is justified when you need more than basic time unit conversion.
Choose ms for straightforward time unit conversions where you're setting timeouts, defining cache expiration, configuring rate limits, or displaying elapsed time in logs. If you find yourself writing code like setTimeout(fn, 24 * 60 * 60 * 1000), replace it with setTimeout(fn, ms('1d')). Don't install date-fns if ms solves your problem—you'll save 17+ KB and avoid unnecessary complexity. These libraries can coexist in the same project serving different needs without conflict.