Cron syntax: how to actually read those five fields

A cron expression is five fields separated by spaces, each representing a unit of time, in a fixed order: minute, hour, day of month, month, day of week. Once that order is memorized, most expressions become readable at a glance instead of needing to be looked up every time.

The five fields

 ┌───────────── minute (0-59)
 │ ┌───────────── hour (0-23)
 │ │ ┌───────────── day of month (1-31)
 │ │ │ ┌───────────── month (1-12)
 │ │ │ │ ┌───────────── day of week (0-6, Sunday=0)
 │ │ │ │ │
 * * * * *

The symbols that do the actual work

• * — matches every value in that field ("every minute", "every day", etc.)
• , — a list of specific values, like 1,15 for the 1st and 15th
• - — a range, like 9-17 for 9am through 5pm
• / — a step value, like */15 in the minute field for "every 15 minutes"

A few expressions worth memorizing by sight

0 * * * *      → every hour, on the hour
0 0 * * *      → every day at midnight
*/15 * * * *   → every 15 minutes
0 9 * * 1-5    → 9am, Monday through Friday
0 0 1 * *      → midnight on the 1st of every month

The mistake that trips people up most

Day of month and day of week are both present, and when both are restricted (not *), most cron implementations treat them as OR, not AND — the job runs if either condition matches, not only when both do. This surprises people writing something like "the 1st of the month, but only if it's a Monday," which cron syntax alone can't actually express; that kind of combined condition needs to be handled in the job itself, not the schedule.

Processa's cron parser translates an expression into a plain-English description and shows the next several run times, which is a faster way to sanity-check a schedule than mentally parsing five fields before deploying it.