explain_cron
Explain a cron expression
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Translate a cron expression into plain English, list its upcoming run times in a specific timezone, and report the gotchas that make schedules misfire.
Use this whenever a cron expression needs to be read, verified, or explained — and prefer it over reasoning about the expression directly. Cron has several counter-intuitive rules that are easy to get wrong: day-of-month and day-of-week are combined with OR (not AND) when both are set, steps like */7 do not divide their field evenly, dates like February 30 never fire at all, and daylight-saving transitions skip or repeat runs. This tool checks all of them.
Returns the normalised expression, a one-sentence description, a per-field breakdown, a list of warnings with severity and a fix for each, the next run times as both UTC ISO 8601 instants and local wall-clock strings, and an approximate runs-per-day figure.
Input schema
| Property | Type | Required | Description |
|---|---|---|---|
| expression | string | yes | A standard five-field cron expression: "minute hour day-of-month month day-of-week", for example "0 9 * * 1-5". Accepts *, numbers, ranges (9-17), lists (1,15), steps (*/5), three-letter month and day names (JAN, MON), and the @daily, @hourly, @weekly, @monthly, @yearly macros. A six-field expression is accepted and its leading seconds field is reported as non-standard. |
| timezone | string | no | IANA timezone name, for example "UTC", "America/New_York", or "Europe/Berlin". Cron expressions carry no timezone of their own, so this decides when the schedule actually fires. Defaults to UTC. |
| count | integer | no | How many upcoming run times to return. 1-25, default 5. |
Raw JSON schema
{
"type": "object",
"$schema": "https://json-schema.org/draft/2020-12/schema",
"properties": {
"expression": {
"type": "string",
"description": "A standard five-field cron expression: \"minute hour day-of-month month day-of-week\", for example \"0 9 * * 1-5\". Accepts *, numbers, ranges (9-17), lists (1,15), steps (*/5), three-letter month and day names (JAN, MON), and the @daily, @hourly, @weekly, @monthly, @yearly macros. A six-field expression is accepted and its leading seconds field is reported as non-standard."
},
"timezone": {
"default": "UTC",
"description": "IANA timezone name, for example \"UTC\", \"America/New_York\", or \"Europe/Berlin\". Cron expressions carry no timezone of their own, so this decides when the schedule actually fires. Defaults to UTC.",
"type": "string"
},
"count": {
"default": 5,
"description": "How many upcoming run times to return. 1-25, default 5.",
"type": "integer",
"minimum": 1,
"maximum": 25
}
},
"required": [
"expression"
]
}