jkbrzt/rrule

rrule.js is a JavaScript and TypeScript library that handles repeating calendar events. Think of events like "every Monday and Friday for the next year" or "the first Tuesday of each month" -- this library lets you define those patterns and then ask it for the actual dates when each occurrence falls. It follows the iCalendar standard, which is the format used by calendar applications like Google Calendar, Apple Calendar, and Outlook when they share or export events. The library was originally adapted from a Python project called python-dateutil, and it brings that same date-calculation logic to the JavaScript world. You install it with npm or yarn, import a few classes into your code, and then build a rule by specifying things like frequency (daily, weekly, monthly), interval (every 2 weeks, every 3 months), which days of the week it applies to, and a start and end date. Once the rule is defined, you can ask for all matching dates, a specific range of dates, or a plain-English description like "every 5 weeks on Monday and Friday until January 31, 2013". For more complex calendaring needs, the library also provides an RRuleSet, which lets you combine multiple repeating rules together, add one-off extra dates, and exclude specific dates or entire repeating patterns from the set. This covers real-world situations where a recurring meeting might be cancelled on a holiday or supplemented with an ad-hoc session. The library includes a parser so you can take a raw iCalendar rule string (the kind you might find inside a .ics calendar file) and turn it back into a working rule object. You can also convert a rule back to that string format, which makes it easy to store rules in a database and reload them later. One area that requires care is how dates are handled. The library works internally with UTC timestamps to avoid confusion from local timezone offsets, and it includes support for named timezones like America/Denver using the browser and Node.js built-in internationalization features. The README includes detailed examples showing the correct and incorrect ways to pass in dates, which matters because JavaScript date handling can produce subtle errors when timezone offsets are applied unexpectedly.