Explain a cron expression

No. The conversion runs entirely in your browser. The file never leaves your device – there is no server that could see it. Once the page has loaded, the tool even works offline.

The classic five fields: minute, hour, day of month, month, weekday. Ranges (1-5), lists (1,3,5), steps (*/15) and names (mon, jan) all work. Deliberately not supported: a seconds column, @daily style macros, and the L/W/# extensions – those differ between cron implementations, and reading them differently from your actual scheduler would be worse than saying so.

That is a genuine cron quirk, not a bug here. When both the day-of-month and the weekday field are restricted, cron fires when *either* matches – so this runs on the 1st of every month and on every Monday. If you want the intersection, restrict only one of the two fields.

Your computer's local time, because that is what a cron daemon uses by default. If your server runs on UTC, its idea of “3 a.m.” will differ from what you see here.