moment/luxon

Luxon is a JavaScript library for working with dates, times, time zones, and durations. It is maintained under the Moment.js organization and is considered a modern successor to Moment.js, addressing several design issues from that older library. The README's opening code example shows the chainable API: you can take the current date and time, shift it to a specific time zone, subtract a week, move to the end of that day, and format the result as an ISO string, all in a single readable chain of method calls. Luxon's key features are its three main types, DateTime (a specific point in time), Duration (an amount of time like "3 hours and 20 minutes"), and Interval (a span of time between two DateTimes), and the fact that these are immutable objects, meaning operations return new objects rather than modifying the original. This prevents a common class of bugs where multiple parts of your code accidentally share and change the same date object. Time zone support is built in using the browser's or Node's native Internationalization (Intl) API, so there are no extra locale or time zone data files to bundle. It also handles parsing and formatting for standard and custom date formats. You would use Luxon when your JavaScript or Node.js application needs to do anything involving dates and times beyond simple comparisons, calculating durations, displaying times in different time zones, parsing dates from strings, or doing date arithmetic. It is MIT-licensed and installed via npm.