Cron expression explainer

It runs every 5 minutes, all day, every day. The */5 is a step in the minute field, so it fires at minute 0, 5, 10, and so on. The four remaining stars mean the hour, day of month, month, and day of week are unrestricted.

Almost always a star where a number belongs. * 3 * * * looks like "3am daily" but it runs every minute for the whole 3am hour, 60 times. The minute field comes first, so nightly at 3am is 0 3 * * *. Checking the next-run list above catches this instantly.

No, and this is cron's strangest rule. When both fields are restricted, the job runs if either matches, not both. So 0 0 1 * 1 fires on the 1st of the month and on every Monday, not only on Mondays that fall on the 1st. When one of them is a star, only the other applies. This tool follows the same rule, which is why the summary says "or".

Both. The day-of-week field accepts 0 to 7, and 0 and 7 are each Sunday. Three-letter names work in most implementations too, so SUN, MON, and the rest are fine. This tool accepts all three forms.

The server's, not yours. If your server runs UTC and you are in Vilnius, a job set for 0 9 * * * lands at noon local in summer. Daylight saving makes it worse: an expression scheduled inside the skipped hour may not run at all on the changeover day, and one inside the repeated hour may run twice. The next-run list above uses your browser's timezone, so treat it as a sanity check on the syntax, not on the server clock.

Not by default. WP-Cron is triggered by page visits, so on a quiet site a "scheduled" task waits for the next visitor and can drift or miss entirely. The usual fix is to disable WP-Cron and have the server's real crontab call wp-cron.php on a fixed schedule, which is where these expressions come in.