How cron next run calculator works
A cron expression has five fields — minute, hour, day-of-month, month and day-of-week — separated by spaces. Each field can be a single value, a list separated by commas, a range with a hyphen, an asterisk meaning every, or a step like */n meaning every nth value. This parser understands all of those forms.
The calculator starts one minute after your chosen start time and walks forward minute by minute, checking the minute, hour and month fields — plus the day-of-month / weekday pair with standard Vixie semantics: when both are restricted the job fires when EITHER matches (OR), matching what real crons do. The first N matches are returned as UTC timestamps. Sunday is weekday 0 (or 7).
Times are computed and returned in UTC, which is the convention most servers use for cron. To preview a schedule locally, read the returned timestamps in your timezone. The tool caps the search at about two years per run, so impossible expressions (like February 30th) report a clear error.