CronSpell

npx add-mcp 'https://cronspell.mike-tusa.workers.dev/mcp'

CronSpell

Type a cron expression to check it, read it in plain English and see when it runs next in your time zone. Daylight saving time is handled: skipped and repeated local times are flagged. Nothing you type is stored.

Syntax and rules

5 fields (minute hour day-of-month month day-of-week) or 6 with a leading seconds field. Each field: *, a value, a range a-b, a list a,b, steps */n, a-b/n, a/n;? in the day fields means *. Month names JAN-DEC and weekday names SUN-SAT (case-insensitive); weekday 0 and 7 are Sunday. Shorthands: @yearly @annually @monthly @weekly @daily @midnight @hourly. If day-of-month and day-of-week are both restricted, a day matches when either matches (classic cron OR rule). Not supported: Quartz L, W and #, 7-field expressions with a year, @reboot, wrap-around ranges such as FRI-MON.

Schedules are read as wall-clock times in the time zone you pass (default UTC). A local time skipped when clocks jump forward does not run (a note says so), and a local time that happens twice when clocks go back is listed twice, each flagged dst "repeated". Cron daemons differ here (cronie, for example, runs fixed-time jobs from a skipped hour right after the jump and only once in a repeated hour), so check the notes.

JSON API

curl -s 'https://cronspell.mike-tusa.workers.dev/api/next?expr=0+9+*+*+1-5&tz=America/Denver&count=5'
curl -s 'https://cronspell.mike-tusa.workers.dev/api/explain?expr=*/15+9-17+*+*+MON-FRI'

Routes: /api/validate, /api/explain, /api/next, /api/previous (up to 20 runs) and /api/between (up to 100 runs). No key needed: about 60 requests per minute per IP. OpenAPI 3.1 spec

For AI agents

CronSpell is also a remote Model Context Protocol (MCP) server with four read-only tools: validate_cron, explain_cron, next_cron_runs and cron_runs_between. Works with AI agents and assistants that can call an MCP server or a JSON API, for example Muse, Claude, ChatGPT or Cursor (descriptive only). Most MCP clients accept this config; some use a different format (for example, VS Code uses a servers key). Clients with a settings screen just need the URL https://cronspell.mike-tusa.workers.dev/mcp.

{ "mcpServers": { "cronspell": { "type": "http", "url": "https://cronspell.mike-tusa.workers.dev/mcp" } } }

Machine-readable docs: llms.txt · OpenAPI 3.1 spec · MCP server card. CronSpell is not affiliated with, endorsed by or sponsored by Meta, Anthropic, OpenAI, Anysphere or any cron software project.