Recurrence Rule (RRULE) Libraries in 2026: rrule.js vs dateutil vs php-rrule vs rrule-go vs rust-rrule

“Every second Tuesday of the month.” “The last Friday before the 15th.” “Weekdays, but never on holidays.” If you have ever tried to express that sentence as a cron expression, you already know the pain: cron cannot do it. Cron handles fixed intervals, not calendar logic. The moment your product needs “the third business day after month-end”, you need a real recurrence engine built on RFC 5545 RRULE — the same standard behind iCalendar, Google Calendar, and every serious scheduling product.

This guide compares the five RRULE libraries that are actually maintained in 2026 across five languages, with live star counts, real API examples pulled from their official repositories, and the pitfalls that will bite you in production.

Quick Verdict

If you are building in JavaScript or TypeScript, use rrule.js — it has the deepest feature set (RRuleSet, natural-language output, caching). In Python, use python-dateutil — it is the reference implementation and ships with the syntax most other tools imitate. In PHP, php-rrule is the only serious option and it is a good one. In Go, rrule-go is pragmatic and fast. In Rust, rust-rrule is the rising option, but check its iterator semantics carefully before you commit.

Rule of thumb: if your scheduling logic has the word “except” or “every other” in it, do not reach for cron. Reach for RRULE.

If you are still running simple periodic jobs, our job scheduling libraries comparison and Linux cron alternatives guide cover that layer. And when the dates that feed your rule arrive as messy strings, start with our datetime parsing libraries roundup.

The Five Libraries Compared

Numbers pulled live from the GitHub API at publish time. Star count is not a quality score — it is a proxy for “how many people will answer your Stack Overflow question.”

Use-Case Decision Matrix

rrule.js — The Reference JavaScript Implementation

Install:

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npm install rrule
# or
yarn add rrule

The core API builds a rule from a plain options object. This example — straight from the project README — produces every fifth week, on Monday and Friday, five times:

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import { RRule } from 'rrule';

const rule = new RRule({
  freq: RRule.WEEKLY,
  interval: 5,
  byweekday: [RRule.MO, RRule.FR],
  dtstart: new Date(Date.UTC(2026, 1, 1, 10, 30)),
  count: 5
});

console.log(rule.all());
console.log(rule.toText()); // "every 5 weeks on Monday and Friday"

What makes rrule.js stand out is RRuleSet, which lets you union several rules and then subtract exceptions. This is exactly how a real booking system expresses “every Monday, except public holidays”:

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import { RRule, RRuleSet, rrulestr } from 'rrule';

const set = new RRuleSet();

// Every Monday
set.rrule(new RRule({ freq: RRule.WEEKLY, byweekday: RRule.MO, count: 10 }));

// ...plus the first of every month
set.rrule(new RRule({ freq: RRule.MONTHLY, bymonthday: 1, count: 3 }));

// ...minus a specific holiday
set.exdate(new Date(Date.UTC(2026, 11, 25)));

console.log(set.all().length);

The one caveat: rule.all() without count or until will throw rather than loop forever — which is a feature, not a bug. Always bound your rules.

python-dateutil — The Reference Implementation

Install:

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pip install python-dateutil

dateutil.rrule is the implementation that most other languages were ported from, and it is the most forgiving about input. You can build rules programmatically:

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from dateutil.rrule import rrule, WEEKLY, MO, FR
from datetime import datetime

rule = rrule(
    WEEKLY,
    interval=5,
    byweekday=(MO, FR),
    dtstart=datetime(2026, 2, 1, 10, 30),
    count=5,
)

for occurrence in rule:
    print(occurrence)

Or — far more useful in practice — parse a raw RFC 5545 string that came from a calendar client:

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from dateutil.rrule import rrulestr

text = "FREQ=WEEKLY;INTERVAL=5;BYDAY=MO,FR;COUNT=5"
rule = rrulestr(text, dtstart=datetime(2026, 2, 1, 10, 30))

print(rule.after(datetime(2026, 3, 1)))   # next occurrence after a date
print(list(rule)[:3])                      # first three occurrences

The after() and before() helpers are the ones you will actually use in production: they let a scheduler ask “what is the next run after now?” without materialising the whole series. The trade-off is that dateutil has no human-readable output — if you need “every 5 weeks on Monday” in the UI, you generate that yourself.

php-rrule — Composer-Native Recurrence

Install:

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composer require rlanvin/php-rrule

The API mirrors the RFC directly, which makes it easy to round-trip values that came from a front-end calendar widget:

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use RRule\RRule;
use RRule\RRuleSet;

$rule = new RRule([
    'FREQ' => 'WEEKLY',
    'INTERVAL' => 5,
    'BYDAY' => ['MO', 'FR'],
    'DTSTART' => new DateTime('2026-02-01 10:30:00'),
    'COUNT' => 5,
]);

foreach ($rule as $occurrence) {
    echo $occurrence->format('Y-m-d H:i:s') . PHP_EOL;
}

// Parse straight from an RFC 5545 string
$fromString = RRule::createFromRfcString('FREQ=WEEKLY;INTERVAL=5;BYDAY=MO,FR');

php-rrule’s RRuleSet and its humanReadable() method give it feature parity with rrule.js — rare for PHP libraries, and the reason it is the default choice for PHP booking engines. Note that humanReadable() output is English-oriented; if you serve multiple locales you will still want your own translation layer.

rrule-go — Iterators for High-Throughput Services

Install:

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go get github.com/teambition/rrule-go

The Go port uses an option struct and returns an iterator, which matters when you are computing thousands of schedules per second:

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package main

import (
	"fmt"
	"time"

	"github.com/teambition/rrule-go"
)

func main() {
	start := time.Date(2026, 2, 1, 10, 30, 0, 0, time.UTC)

	r, err := rrule.NewRRule(rrule.ROption{
		Freq:      rrule.WEEKLY,
		Interval:  5,
		Byweekday: []rrule.Weekday{rrule.MO, rrule.FR},
		Dtstart:   start,
		Count:     5,
	})
	if err != nil {
		panic(err)
	}

	for _, d := range r.All() {
		fmt.Println(d)
	}

	// Or stream without building a slice:
	it := r.Iterator()
	for it.Next() {
		fmt.Println(it.Value())
	}
}

The Iterator() path is the one to use in services: All() allocates a slice for the entire series, while the iterator stops as soon as you break out of the loop. rrule-go also exposes Set for union/exclusion, though its timezone story is thinner than rrule.js — it works best when you normalise everything to UTC at the edge.

rust-rrule — Native Iterators in the Rust Ecosystem

Install:

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cargo add rrule

The Rust implementation leans on chrono types and implements Iterator directly:

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use chrono::{TimeZone, Utc};
use rrule::{Frequency, RRule, RRuleProperties, Weekday};

fn main() {
    let start = Utc.with_ymd_and_hms(2026, 2, 1, 10, 30, 0).unwrap();

    let props = RRuleProperties {
        freq: Frequency::Weekly,
        interval: 5,
        by_weekday: vec![
            Weekday::Mon.into(),
            Weekday::Fri.into(),
        ],
        dt_start: start.into(),
        count: Some(5),
        ..Default::default()
    };

    let rule = RRule::new(props).expect("valid rule");

    for date in rule.into_iter().take(5) {
        println!("{date}");
    }
}

Be deliberate about which entry point you use: RRule::new() sets up validation, while the iterator variants differ in whether dt_start itself is yielded first. If your first generated date is off by one, that is the knob to check — not your timezone conversion.

Common Pitfalls and Migration Notes

1. DST will silently shift your wall-clock times. FREQ=DAILY;BYHOUR=9 should fire at 09:00 local time every day. If you build the rule in UTC with a fixed HH:MM, it will drift to 08:00 or 10:00 twice a year. Always anchor DTSTART in the target timezone (via tzid in rrule.js, tzinfo in dateutil, DateTimeZone in PHP, chrono-tz in Rust) rather than converting UTC yourself.

2. Unbounded rules are a production incident waiting to happen. Never call all() without count or until. Prefer after()/before()/between() so the computation is bounded by your query window.

3. EXDATE removes occurrences; RDATE adds them. They are not symmetric with RRULE. If you are storing “skipped meetings”, append to EXDATE. If you are storing “extra one-off sessions”, append to RDATE — never encode one-off dates by editing the base rule.

4. BYSETPOS is how you express ordinals. “Last Friday of the month” is FREQ=MONTHLY;BYDAY=FR;BYSETPOS=-1. Reaching for INTERVAL will not get you there.

5. Migration trap. If you are porting from cron to RRULE, there is no automatic mapping: cron’s step syntax (*/5) has no ordinal equivalent, and RRULE’s BYDAY semantics differ from cron’s day-of-week field. Write a mapping table for your existing schedules and test both engines side by side for a full month before you cut over.

FAQ

Which RRULE library should I use in JavaScript? Use rrule.js. It has the most complete RFC 5545 coverage among JavaScript options, including RRuleSet, EXDATE/RDATE, and human-readable output through toText().

Can I use cron instead of RRULE? Only if your schedule is a fixed interval. Cron cannot express ordinals like “the third Tuesday” or “the last business day of the month.” That is exactly the class of problem RRULE exists to solve.

Does python-dateutil support parsing RFC 5545 strings? Yes. rrulestr() accepts a raw rule string such as FREQ=WEEKLY;INTERVAL=5;BYDAY=MO,FR;COUNT=5 and returns a callable rule object, which makes it ideal for consuming calendar data from external clients.

How do I exclude holidays from a recurrence rule? Add the dates to the rule’s exclusion list: set.exdate(...) in rrule.js, exdate in dateutil and php-rrule, and the exclusion helpers on Go’s Set and Rust’s rule set. Do not try to bend BYDAY into expressing holidays.

Why does my rule generate an infinite loop? Because you did not bound it. A rule with neither COUNT nor UNTIL is infinite by definition — most libraries expose a bounded after()/before() API specifically so you never materialise the whole series.

How do I get “last Friday of the month” in RRULE? Use BYSETPOS=-1 together with BYDAY=FR: FREQ=MONTHLY;BYDAY=FR;BYSETPOS=-1. Negative set positions count backwards from the end of the period.

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