Developer tools
How to read a cron expression
A line like 0 9 * * 1 is easy to misread. Paste it and check each field before you put it on a server.
Five fields, or six when seconds come first
A standard expression is minute, hour, day of month, month, and day of week. Some schedulers add seconds at the front. The explainer accepts both and names each field.
A star means every value in that field. A list, a range, and a step are read out in words so you can see whether Monday at 09:00 is what you meant.
Day of month and day of week together
If both of those fields are restricted, many schedulers run the job when either one matches. Read that line twice before you rely on it for a once-a-month task.
The expression stays on the page
Nothing is sent to a server. The explainer describes the schedule. It does not install it.