Because I cannot remember the correct syntax for cron, I found this little extension for VScode:
https://marketplace.visualstudio.com/items?itemName=tumido.cron-explained
Just hover over the cron expression and you see if defined the correct time for your rule.
I have found that extension some weeks ago and added an issue for the openHAB extension, to include this in the openHAB extension too.
Reference:
There is already a javascript package existing, which should be able to integrate with a fair amount of work.
So if someone wants to start with that, i am happy to help. 
