Back to catalog
RRuleScheduler
MITRRuleScheduler evaluates RFC 5545 calendar recurrence rules in JavaScript and TypeScript, calculating repeating schedules without recurring software fees.
Overview
Czech software engineer Jakub Roztocil created rrule.js to port recurrence arithmetic from Python's dateutil library to JavaScript. Over a decade, it became the standard TypeScript implementation of iCalendar RFC 5545 recurrence rules for browsers and Node.js. Today, it serves as an algorithmic utility rather than an end-user booking portal, providing an embeddable date math engine that runs within application code.
Where RRuleScheduler fits
Unlike hosted appointment platforms with booking interfaces and calendar sync, RRuleScheduler functions strictly as a date calculation engine. Applications use it to evaluate recurring event definitions on demand rather than storing thousands of individual rows in a database. Computing event occurrences in memory or in the browser eliminates the storage inflation and synchronization issues that accompany pre-generated calendar series.
This architecture minimizes operational overhead while shifting workflow design to the developer. RRuleScheduler eliminates recurring software subscription costs, executing deterministic arithmetic with zero database connections or daemons. The trade-off is development scope: engineering teams must build their own booking interfaces, reservation logic, and conflict checks, pairing the recurrence engine with custom application code.
What it does well
- RFC 5545 recurrence arithmetic — parses and outputs standard iCalendar recurrence rule strings, ensuring interoperability with external calendar services like Google Calendar and Apple Calendar.
- Dynamic occurrence querying — provides between, before, and after methods to generate event dates on demand across any designated time window.
- Natural language serialization — converts recurrence rules to and from English phrases via text parsing methods, simplifying user-facing rule display in web applications.
- Complex set combinations — supports rule set definitions that combine multiple recurrence rules, explicit inclusion dates, and exclusion dates into a unified evaluation pipeline.
- Universal JavaScript runtime support — executes across Node.js backends and browser runtimes without native dependencies, enabling identical recurrence logic in frontend previews and backend workers.
- Native TypeScript definitions — ships built-in type declarations that provide compile-time option validation and autocompletion for recurrence intervals, frequencies, and weekday constants.
