Cron not working on the correct date

Description

I want to trigger a method every first monday of the month. (this can be configured that it is every x months)

So for doing this, i had to use the following cron: 0 0 */31,1-7 */1 MON Looking at: https://elmah.io/tools/cron-parser/#0_0_*/31,1-7_*/1_1 this works as expected

Expected Behavior

Next date to be: Mon Jan 01 2024 01:00:00 GMT+0100

Actual Behavior

Date is the next monday (not first monday of the year): 2023-12-25T00:00:00.000+01:00

Possible Fix

No response

Steps to Reproduce

Link to stackbllitz

import * as cron from 'cron';

const dt = cron.sendAt('0 0 */31,1-7 */1 MON');
console.log(`The job would run at ...: ${dt.toISO()}`);

Context

The current date is: 22 december

Your Environment

cron version: 3.1.6
NodeJS version: 20.10.0

I'm not a maintainer of this library, but let's break this down:

Part	Expression	Interpreted as ...
Minutes	`0`	at minute 00
Hours	`0`	at hour 00
Day of month	`*/31,1-7`	Every 31st day, starting from and including the 1st in every month and also day 1-7. Essentially days 1,2,3,4,5,6,7 (and 32 if it would have been a match).
Month	`*/1`	Every month, the same as a simple `*`
Day of week	`MON`	At mondays

As cron expressions do not combine the day of the month and the day of the week, any match in either of these fields would make a match. In this case, Mondays AND 1st, 2nd, 3rd, 4th, 5th, 6th, 7th. So the correct series of matches from today (2023-12-24) would be:

2023-12-25 00:00:00
2024-01-01 00:00:00
2024-01-02 00:00:00
2024-01-03 00:00:00
2024-01-04 00:00:00
2024-01-05 00:00:00
2024-01-06 00:00:00
2024-01-07 00:00:00
2024-01-08 00:00:00
2024-01-15 00:00:00
2024-01-22 00:00:00
2024-01-29 00:00:00
2024-02-01 00:00:00
2024-02-02 00:00:00
2024-02-03 00:00:00

This means your reference site " elmah.io " is interpreting this pattern incorrectly. In addition to that, I do not think this library has any way to match the first weekday of the month, so you'll either have to make this a feature request or look for a library that supports the 'nth weekday' specifier. In your case, using a library that supports this feature, you could use a pattern such as '0 0 MON#1' to achieve 00:00 at the first Monday of every month."

kelektiv / node-cron