Cron Expression Builder

The builder emits standard 5-field Unix cron only: minute (0-59), hour (0-23), day-of-month (1-31), month (1-12), and day-of-week (0-6, where 0=Sun and 6=Sat). There is no seconds field and no year field, so expressions here are not directly compatible with Quartz/Spring (Java) or other 6-7 field dialects, and the day-of-week numbering does not accept 7 as Sunday the way Vixie cron does. Each field accepts four operators, listed in the Quick Reference: '*' (any value), ',' (a list such as 0,6), '-' (a range such as 1-5), and '/' (a step such as */15). Step ranges like 1-10/2 are also parsed. Typing into the single expression box and into the five per-field boxes stay in sync, and the parser splits on whitespace and requires exactly five parts, anything with more or fewer fields is silently reset to '* * * * *' rather than flagged as an error.

The 'Next Scheduled Runs' panel is computed by brute-force simulation rather than a cron library: it starts at the current minute and steps forward one minute at a time, testing each candidate against the parsed fields until it collects 5 matches or hits a cap of 525,600 iterations (one year of minutes). If no run is found within that year, the panel shows 'Invalid cron expression - no scheduled runs found'. One behavior to know is that this matcher ANDs the day-of-month and day-of-week fields together (both must match), which differs from traditional Vixie cron, where if both day fields are restricted the job fires when EITHER matches. Run times are rendered in your browser's local timezone with a relative countdown (in 2h 15m, in 3d 4h), and the matcher honors local-time semantics, so a schedule like 0 9 * * 1-5 evaluates 9 AM in your system zone, not UTC.

Above the runs, a plain-English description is generated from the fields, recognizing patterns like */N ("every N minutes/hours"), 1-5 ("on weekdays"), and 0,6 ("on weekends"), and translating hours into 12-hour AM/PM. Ten presets are provided as one-click buttons (every minute, every hour, midnight, noon, every Monday 9 AM, every weekday 9 AM, every 15 and 30 minutes, first of month, and Sunday 6 PM), and the active preset is highlighted when the current expression matches it exactly. The default loaded expression is 0 9 * * 1-5 (weekdays at 9 AM). The expression is stored in the page URL via shareable state, so a Share button produces a link that reopens the builder with the same schedule, and a Copy button puts the raw expression on the clipboard. Bottom-of-page legends spell out the 0-6 weekday and 1-12 month numbering for reference.