Recurrence Validator

Overview

Parses an RFC 5545 recurrence rule with ical4j and reports whether it is valid, what it means in a fixed English rendering, which parts it declares, and up to 10 preview occurrences. Structural observations are returned as findings: a BYMONTHDAY that skips short months, a BYYEARDAY=366 that skips common years, a BYWEEKNO whose meaning depends on an undeclared WKST, and a rule with no COUNT or UNTIL. A malformed rule is a finding with valid: false, not an error, because evaluating the rule succeeded. This validates rule syntax and structure — it does not judge whether the schedule is the one you intended.

A valid RRULE can still skip months or repeat forever — we parse with ical4j, surface structural findings, and preview a few occurrences before you materialize the full series.

When to use it

Scope

Non-goals

• Does not expand a full series (use time.generate_recurrence)
• Does not author a rule from a natural-language description
• Does not correct an invalid rule or suggest a replacement
• Does not judge whether the schedule matches the caller's intent — only its syntax and structure
• Does not read the system clock

Properties

Capabilities

Cost & billing

Flat rate per invocation

File handling

How we handle your data

Input schema

Output schema

API contract

curl -sS -X POST "$BASE/v1/execute" -H "Authorization: Bearer $KEY" -H 'Content-Type: application/json' -d '{"skillKey":"time.validate_recurrence","input":{"rrule":"FREQ=MONTHLY;BYMONTHDAY=31","startDateTime":"2026-01-31T09:00:00","zone":"UTC"}}'

import { Squillify } from "@squillify/sdk";

const sq = new Squillify({ apiKey: process.env.SQUILLIFY_API_KEY });

const { output } = await sq.execute("time.validate_recurrence", {"zone":"UTC","startDateTime":"2026-01-31T09:00:00","rrule":"FREQ=MONTHLY;BYMONTHDAY=31","previewCount":3});

Errors

Other limits

Limitations

• Does not expand a full series (use time.generate_recurrence)
• Does not author a rule from a natural-language description
• Does not correct an invalid rule or suggest a replacement
• Does not judge whether the schedule matches the caller's intent — only its syntax and structure
• Does not read the system clock
• Materializing a long series (use time.generate_recurrence)
• Parsing an .ics file
• Turning prose into a rule

How to call

POST /v1/execute (requires authentication)

{
  "skillKey": "time.validate_recurrence",
  "input": {
    "zone": "UTC",
    "startDateTime": "2026-01-31T09:00:00",
    "rrule": "FREQ=MONTHLY;BYMONTHDAY=31",
    "previewCount": 3
  }
}

Examples

{
  "zone": "UTC",
  "startDateTime": "2026-01-31T09:00:00",
  "rrule": "FREQ=MONTHLY;BYMONTHDAY=31",
  "previewCount": 3
}

{
  "explanation": "Every month on day 31",
  "findings": [
    {
      "code": "RRULE_SKIPS_SHORT_MONTHS",
      "message": "BYMONTHDAY=31 yields no occurrence in months that are too short.",
      "path": "/rrule",
      "severity": "warning",
      "details": {
        "monthDays": [
          "31"
        ]
      }
    },
    {
      "code": "RRULE_UNBOUNDED",
      "message": "The rule declares neither COUNT nor UNTIL and therefore repeats indefinitely.",
      "path": "/rrule",
      "severity": "info",
      "details": {}
    }
  ],
  "valid": true,
  "nextOccurrences": [
    "2026-01-31T09:00:00Z",
    "2026-03-31T09:00:00Z",
    "2026-05-31T09:00:00Z"
  ],
  "parts": {
    "BYMONTHDAY": "31",
    "FREQ": "MONTHLY"
  },
  "infinite": true,
  "_meta": {
    "engine": "ical4j",
    "engineVersion": "4.3.0",
    "tzdbVersion": "2026b"
  },
  "normalized": "FREQ=MONTHLY;BYMONTHDAY=31"
}

{
  "startDateTime": "2026-03-02T09:00:00",
  "zone": "UTC",
  "rrule": "FREQ=WEEKLY;BYDAY=XX"
}

{
  "findings": [
    {
      "code": "RRULE_MALFORMED",
      "message": "Part 'BYDAY' has a value the RFC 5545 grammar does not accept.",
      "path": "/rrule",
      "severity": "error",
      "details": {}
    }
  ],
  "_meta": {
    "engine": "ical4j",
    "engineVersion": "4.3.0",
    "tzdbVersion": "2026b"
  },
  "valid": false,
  "explanation": "",
  "nextOccurrences": [],
  "infinite": false,
  "parts": {}
}

Recommended use cases

• Check a user-entered recurrence rule before storing it
• Show a human-readable summary of a stored rule in a UI
• Discover that BYMONTHDAY=31 silently skips February, April, June, September, and November

Avoid for

• Materializing a long series (use time.generate_recurrence)
• Parsing an .ics file
• Turning prose into a rule

Related skills

Composable with

Safety & privacy

• Runs without calling any external service.
• Your payloads are never stored permanently.
• Telemetry captures metadata only — never your payload contents.
• We never train models on your payloads.
• No human reviews your payloads.

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