Cron expressions
How cron syntax works, and the gotchas that trip people up
A standard cron expression has five fields, in order: minute (0-59), hour (0-23), day of month (1-31), month (1-12), and day of week (0-6, where both 0 and 7 usually mean Sunday depending on the implementation). Each field accepts a literal value, a wildcard `*` for "every," a range like `1-5`, a list like `MON,WED,FRI`, or a step like `*/15` for "every 15 units starting from the field's minimum." So `*/15 * * * *` runs at :00, :15, :30, and :45 of every hour — not "every 15 minutes from whenever the schedule was created."
The field people get wrong most often is the interaction between day-of-month and day-of-week: in standard cron, if BOTH fields are restricted (neither is `*`), the schedule fires when EITHER condition is true, not when both are — an OR, not an AND. So `0 0 15 * MON` doesn't mean "the 15th, if it's a Monday"; it means "every 15th of the month, AND every Monday."
Not all cron dialects are identical. Quartz (common in Java scheduling frameworks) adds a seconds field and a year field, uses 1-7 for day-of-week starting at Sunday=1, and — unlike standard cron — requires you to put `?` in one of day-of-month or day-of-week to make the OR ambiguity explicit. AWS EventBridge's cron is Quartz-derived and has the same day-of-month/day-of-week restriction. Reading someone else's cron expression cold, or writing one for a system you don't touch often, is exactly the kind of thing that's easy to get subtly wrong — which is why Cronwright's visual builder and plain-English preview exist: build it visually, or paste one to see in plain language when it will actually run, across Standard, Quartz, and EventBridge dialects.
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Frequently asked questions
What do the five fields in a cron expression mean?
In order: minute (0-59), hour (0-23), day of month (1-31), month (1-12), and day of week (0-6 or 1-7 depending on the system). Each field can be a specific value, a wildcard `*`, a range (`1-5`), a list (`MON,WED,FRI`), or a step (`*/15`).
Why does 0 0 15 * MON run on both the 15th and every Monday, not just Mondays that fall on the 15th?
Standard cron treats day-of-month and day-of-week as an OR when both are restricted, not an AND. If you actually want "the 15th, only if it's a Monday," cron alone can't express that — you'd need an external check or a scheduler dialect (like Quartz) that lets you set one of the two fields to a non-conflicting placeholder.
What's different about Quartz cron compared to standard cron?
Quartz adds a seconds field (making it six or seven fields total, with an optional year), numbers day-of-week 1-7 starting from Sunday, and requires a `?` placeholder in either the day-of-month or day-of-week field to resolve the ambiguity that standard cron leaves implicit. AWS EventBridge's cron syntax is derived from Quartz and shares this rule.
Does */15 * * * * mean 'every 15 minutes from now'?
No — it means every 15 minutes on the clock, aligned to the hour: :00, :15, :30, :45. It doesn't shift based on when the job was scheduled or last ran.