# CRN0

> CRN0 (https://crn0.com) converts cron expressions between timezones. It is DST-aware, explains schedules in plain English and lists the next run times. It is free, needs no auth, and offers an MCP server and a JSON API for agents.

Use CRN0 when a schedule is written for one timezone (e.g. "9am Sydney on weekdays") but runs on a machine or scheduler set to another timezone (usually UTC: GitHub Actions, AWS EventBridge, Kubernetes CronJobs, Vercel, Cloudflare).

Key facts for agents:
- Cron format: 5 fields (minute hour day-of-month month day-of-week). Names (JAN, MON), macros (@daily, @hourly...), and * , - / ? are supported. Seconds, year, L, W and # are not.
- A converted schedule can need more than one cron line (e.g. "every 15 min, 9am-5pm" across a day boundary). Use every returned line.
- Conversions use the UTC offsets that apply on the reference date. If either zone observes DST, the converted cron drifts by an hour for part of the year. Fixed-offset schedulers can't avoid this, so the result includes a `dstNote`.
- Timezone inputs accept IANA ids, abbreviations (PST, AEST, IST), offsets (UTC+8) or city names.

## Agent interfaces

- [MCP server](https://crn0.com/mcp): Streamable HTTP endpoint with no auth. Tools: convert_cron, describe_cron, next_cron_runs, search_timezones
- [JSON API: convert](https://crn0.com/api/convert?cron=0+9+*+*+1-5&from=Australia/Sydney&to=UTC): GET or POST. Params: cron, from, to, at?, count?
- [JSON API: describe](https://crn0.com/api/describe?cron=*/15+9-17+*+*+1-5): plain-English explanation and expanded fields
- [JSON API: next runs](https://crn0.com/api/next-runs?cron=0+0+*+*+*&timezone=America/New_York&count=5): upcoming run times
- [JSON API: timezone search](https://crn0.com/api/timezones?query=melbourne): resolves a city or abbreviation to an IANA id
- [OpenAPI spec](https://crn0.com/openapi.json): OpenAPI 3.1 description of the JSON API
- [Agent docs page](https://crn0.com/agents): setup instructions for Claude, Cursor, VS Code and other MCP clients

## Human pages

- [Converter](https://crn0.com/): interactive converter. Deep link: https://crn0.com/?cron=<cron>&from=<IANA>&to=<IANA>
- [Timezone index](https://crn0.com/timezones): every supported timezone with offsets and DST rules
- Pair pages, for example [UTC to EST](https://crn0.com/utc-to-est), [UTC to PST](https://crn0.com/utc-to-pst), [UTC to CST](https://crn0.com/utc-to-cst), [UTC to MST](https://crn0.com/utc-to-mst). Pattern: https://crn0.com/<from>-to-<to> using the slugs in llms-full.txt

## Optional

- [Full reference](https://crn0.com/llms-full.txt): tool schemas, API examples and every timezone slug
- [Sitemap](https://crn0.com/sitemap.xml)

---

# Full reference

## MCP server

Endpoint: https://crn0.com/mcp. Transport: Streamable HTTP (stateless, JSON responses). Auth: none.

Client config (Claude Desktop, Cursor, Windsurf, VS Code and others):

```json
{
  "mcpServers": {
    "crn0": {
      "type": "http",
      "url": "https://crn0.com/mcp"
    }
  }
}
```

Claude Code: `claude mcp add --transport http crn0 https://crn0.com/mcp`

Raw JSON-RPC example:

```bash
curl -s https://crn0.com/mcp -H 'Content-Type: application/json' -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"convert_cron","arguments":{"cron":"0 9 * * 1-5","from":"Australia/Sydney","to":"UTC"}}}'
```

## Tools

### convert_cron

Convert a cron expression from one timezone to another so it fires at the same real-world instants. Returns the converted cron line(s), plain-English descriptions, offsets, warnings (day shifts, multi-line results, DST) and the next run times in both zones.

Input schema:

```json
{
  "type": "object",
  "properties": {
    "cron": {
      "type": "string",
      "description": "Standard 5-field cron, e.g. \"0 9 * * 1-5\", or a macro like \"@daily\"."
    },
    "from": {
      "type": "string",
      "description": "Timezone the schedule is written in (IANA id, abbreviation, offset or city). Default \"UTC\"."
    },
    "to": {
      "type": "string",
      "description": "Timezone of the machine/scheduler that will run it, e.g. \"UTC\" for most cloud schedulers."
    },
    "at": {
      "type": "string",
      "description": "ISO 8601 date used to evaluate DST offsets. Defaults to now."
    },
    "count": {
      "type": "integer",
      "minimum": 1,
      "maximum": 50,
      "description": "How many upcoming runs to list (default 3)."
    }
  },
  "required": [
    "cron",
    "to"
  ]
}
```

### describe_cron

Validate a cron expression and explain it in plain English, with the expanded values of each field.

Input schema:

```json
{
  "type": "object",
  "properties": {
    "cron": {
      "type": "string",
      "description": "Standard 5-field cron, e.g. \"0 9 * * 1-5\", or a macro like \"@daily\"."
    }
  },
  "required": [
    "cron"
  ]
}
```

### next_cron_runs

List the next run times of a cron expression evaluated in a given timezone (DST-aware), in UTC and local wall time.

Input schema:

```json
{
  "type": "object",
  "properties": {
    "cron": {
      "type": "string",
      "description": "Standard 5-field cron, e.g. \"0 9 * * 1-5\", or a macro like \"@daily\"."
    },
    "timezone": {
      "type": "string",
      "description": "Timezone the cron is evaluated in. Default \"UTC\"."
    },
    "count": {
      "type": "integer",
      "minimum": 1,
      "maximum": 50,
      "description": "Number of runs (default 5)."
    },
    "from": {
      "type": "string",
      "description": "ISO 8601 start instant. Defaults to now."
    }
  },
  "required": [
    "cron"
  ]
}
```

### search_timezones

Look up IANA timezone ids and current UTC offsets by city, country, abbreviation or IANA name.

Input schema:

```json
{
  "type": "object",
  "properties": {
    "query": {
      "type": "string",
      "description": "e.g. \"Melbourne\", \"IST\", \"Europe/\""
    },
    "limit": {
      "type": "integer",
      "minimum": 1,
      "maximum": 50,
      "description": "Max results (default 10)."
    }
  },
  "required": [
    "query"
  ]
}
```

## JSON API

Every endpoint accepts GET query params or a POST JSON body. CORS is open. Errors return HTTP 400 with `{"error": "..."}`.

```bash
curl 'https://crn0.com/api/convert?cron=0+9+*+*+1-5&from=Australia/Sydney&to=UTC'
```

Response (abridged):

```json
{
  "input":  { "cron": "0 9 * * 1-5", "description": "At 09:00 on Monday through Friday.", "timezone": { "iana": "Australia/Sydney", "label": "AEST", "offset": "UTC+11" } },
  "output": { "crons": ["0 22 * * 0-4"], "descriptions": ["At 22:00 on Sunday through Thursday."], "timezone": { "iana": "UTC", "label": "UTC", "offset": "UTC+0" } },
  "deltaMinutes": -660,
  "dayShifts": [-1],
  "warnings": [],
  "dstNote": "The offset between these timezones changes during the year because of daylight saving time...",
  "nextRuns": [{ "utc": "...", "source": "...", "target": "..." }],
  "url": "https://crn0.com/?cron=0+9+*+*+1-5&from=Australia%2FSydney&to=UTC"
}
```

## Common schedules

- `0 9 * * *`: Every day at 9am
- `0 9 * * 1-5`: Weekdays at 9am
- `*/15 * * * *`: Every 15 minutes
- `0 0 * * *`: Every day at midnight
- `30 2 * * 0`: Sundays at 2:30am
- `0 0 1 * *`: First of every month
- `0 */6 * * *`: Every 6 hours
- `0 17 * * 5`: Fridays at 5pm

## Timezones with landing pages

Use the slug in pair URLs: https://crn0.com/<from-slug>-to-<to-slug>. Any IANA timezone also works with the API, MCP tools and the converter.

| slug | abbr | name | IANA | offset |
|---|---|---|---|---|
| utc | UTC | Coordinated Universal Time | UTC | UTC+0 |
| gmt | GMT | Greenwich Mean Time (UK) | Europe/London | UTC+0 / UTC+1 (DST) |
| est | EST | Eastern Time | America/New_York | UTC-5 / UTC-4 (DST) |
| cst | CST | Central Time | America/Chicago | UTC-6 / UTC-5 (DST) |
| mst | MST | Mountain Time | America/Denver | UTC-7 / UTC-6 (DST) |
| pst | PST | Pacific Time | America/Los_Angeles | UTC-8 / UTC-7 (DST) |
| akst | AKST | Alaska Time | America/Anchorage | UTC-9 / UTC-8 (DST) |
| hst | HST | Hawaii Time | Pacific/Honolulu | UTC-10 |
| ast | AST | Atlantic Time | America/Halifax | UTC-4 / UTC-3 (DST) |
| brt | BRT | Brasília Time | America/Sao_Paulo | UTC-3 |
| art | ART | Argentina Time | America/Argentina/Buenos_Aires | UTC-3 |
| wet | WET | Western European Time | Europe/Lisbon | UTC+0 / UTC+1 (DST) |
| cet | CET | Central European Time | Europe/Paris | UTC+1 / UTC+2 (DST) |
| eet | EET | Eastern European Time | Europe/Athens | UTC+2 / UTC+3 (DST) |
| msk | MSK | Moscow Time | Europe/Moscow | UTC+3 |
| sast | SAST | South Africa Standard Time | Africa/Johannesburg | UTC+2 |
| wat | WAT | West Africa Time | Africa/Lagos | UTC+1 |
| eat | EAT | East Africa Time | Africa/Nairobi | UTC+3 |
| gst | GST | Gulf Standard Time | Asia/Dubai | UTC+4 |
| pkt | PKT | Pakistan Standard Time | Asia/Karachi | UTC+5 |
| ist | IST | India Standard Time | Asia/Kolkata | UTC+5:30 |
| npt | NPT | Nepal Time | Asia/Kathmandu | UTC+5:45 |
| ict | ICT | Indochina Time | Asia/Bangkok | UTC+7 |
| sgt | SGT | Singapore Time | Asia/Singapore | UTC+8 |
| hkt | HKT | Hong Kong Time | Asia/Hong_Kong | UTC+8 |
| china | CST | China Standard Time | Asia/Shanghai | UTC+8 |
| pht | PHT | Philippine Time | Asia/Manila | UTC+8 |
| awst | AWST | Australian Western Time | Australia/Perth | UTC+8 |
| jst | JST | Japan Standard Time | Asia/Tokyo | UTC+9 |
| kst | KST | Korea Standard Time | Asia/Seoul | UTC+9 |
| acst | ACST | Australian Central Time | Australia/Adelaide | UTC+9:30 / UTC+10:30 (DST) |
| aest | AEST | Australian Eastern Time | Australia/Sydney | UTC+10 / UTC+11 (DST) |
| brisbane | AEST | Brisbane Time (Queensland) | Australia/Brisbane | UTC+10 |
| nzst | NZST | New Zealand Time | Pacific/Auckland | UTC+12 / UTC+13 (DST) |