Bug?: Aws eventbridge scheduler seems to have a bug

Question

In cron expression
cron (30 4 31W * ? *)
The next 10 trigger dates are

```
Wed, 31 Dec 2025 04:30:00 (UTC + 05:30)
Fri, 30 Jan 2026 04:30:00 (UTC + 05:30)
-- skips feb 2026 (which is correct, it skips all months containing 30 or 28 or 29 days)
Tue, 31 Mar 2026 04:30:00 (UTC + 05:30)
Fri, 31 Jul 2026 04:30:00 (UTC + 05:30). -- where did may go, may 31 2026 is on sun, it should give 29 may 2026
Mon, 31 Aug 2026 04:30:00 (UTC + 05:30)
Thu, 31 Dec 2026 04:30:00 (UTC + 05:30).  -- where did october go , oct 31st is on sat, it should give 30th oct2026
Fri, 29 Jan 2027 04:30:00 (UTC + 05:30)
...
....
....
```

In cron expression
cron (30 4 15/5,22W * ? *). -- actually any number in front of W
The trigger dates it gives is

```
Fri, 14 Nov 2025 04:30:00 (UTC + 05:30)      -- looks like trying to honor weekdays
Mon, 15 Dec 2025 04:30:00 (UTC + 05:30)
Thu, 15 Jan 2026 04:30:00 (UTC + 05:30)
Mon, 16 Feb 2026 04:30:00 (UTC + 05:30)   -- same here, trying to honor weekdays
....
....
```

However,
In cron expression
cron (30 4 15/5,22W,27 * ? *) -- this is accepted by the way by the cron expression in aws console

The next trigger dates are

```
Sat, 15 Nov 2025 04:30:00 (UTC +05:30)     -- why from here first of all, shouldn't it be 14th like previous one
Thu, 20 Nov 2025 04:30:00 (UTC + 05:30)
Sat, 22 Nov 2025 04:30:00 (UTC + 05:30)    -- where did this come from ?
Tue, 25 Nov 2025 04:30:00 (UTC + 05:30)
....
....
....
```
and in the previous one it skipped the step of 5 within the month, where has here it is observing it

Do tell, i am guessing these are bugs
May be a seasoned eye can look at it and tell me what's the issue here and how should i be prepared for this seemingly erratic behaviour

Shahed Kazi (@shajam) · Answer

`cron(30 4 31W * ? *)` runs on the nearest weekday to 31st. Months that don't have 31st will be skipped. If your intention is to run on the last weekday of the month, change it to `cron(30 4 LW * ? *)`.

`cron(30 4 15/5,22W * ? *)` means
- Every month’s 15, 20, 25, 30 and
- The weekday nearest to the 22nd (so if the 22nd falls on a weekend, it adjusts to Friday or Monday).

So, the key question is why the dates are different after adding 27th. You are correct that it should not change the other dates. It’s due to how AWS interprets combined day-of-month expressions, especially when mixed with “W” (weekday-nearest) and increments (like `/”). You should raise an AWS Support ticket and ask for detail explanation.

re:Post Agent · Answer

The behavior you're observing with AWS EventBridge Scheduler's cron expressions isn't necessarily a bug, but rather reflects the complexity of how the W character (weekday nearest) works in combination with other cron expression elements.

For your first expression `cron(30 4 31W * ? *)`:
The W modifier means "the nearest weekday to the given day." When you specify 31W, it's looking for the nearest weekday to the 31st of each month. If a month doesn't have 31 days, it skips that month entirely rather than finding the nearest weekday to the last day of that month. For May 2026, May 31 falls on a Sunday, so the scheduler should select Friday, May 29 (the nearest weekday). Similarly, for October 2026, October 31 falls on a Saturday, so it should select Friday, October 30. The fact that these dates are missing suggests there might be an issue with how the scheduler is handling the W modifier for specific months.

For your second expression `cron(30 4 15/5,22W * ? *)`:
This is a complex expression combining multiple day-of-month selectors. The 15/5 means "starting at day 15, every 5 days" and 22W means "the weekday nearest to day 22." The scheduler appears to be correctly adjusting to weekdays when the specified date falls on a weekend.

For your third expression `cron(30 4 15/5,22W,27 * ? *)`:
Adding the 27th day to the mix changes the behavior. The scheduler now seems to be treating the 15/5 part differently, not adjusting for weekdays as it did in the second expression. The appearance of November 22 makes sense because of the 22W part, but the overall behavior does seem inconsistent with the previous expression.

These inconsistencies suggest that there might be some edge cases in how EventBridge Scheduler interprets complex cron expressions, especially when combining the W modifier with other selectors. For critical scheduling needs, I would recommend:

1. Use simpler cron expressions when possible
2. Always verify the next trigger dates shown in the console
3. Consider implementing additional validation in your application logic
4. For complex scheduling requirements, you might need to create multiple simpler schedules instead of one complex one

If these behaviors are causing issues for your application, you may want to contact AWS Support for clarification on the expected behavior.  
**Sources**  
[Bug?: AWS EventBridge Rate based scheduler Ignores End Date | AWS re:Post](https://repost.aws/questions/QUUoBcFO4qTXOAqrUUtKJRUw/bug-aws-eventbridge-rate-based-scheduler-ignores-end-date)  
[AWS EventBridge Scheduler Rules Fire Immediately After Terraform Deployment Outside Scheduled Window | AWS re:Post](https://repost.aws/questions/QU5sT3FKMQSyez3KI1gcgzBw/aws-eventbridge-scheduler-rules-fire-immediately-after-terraform-deployment-outside-scheduled-window)  
[What is Amazon EventBridge Scheduler? - EventBridge Scheduler](https://docs.aws.amazon.com/scheduler/latest/UserGuide/what-is-scheduler.html)

Bug?: Aws eventbridge scheduler seems to have a bug

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