Here's my version of a Cron format date and time generator for use with KM's Cron Trigger. As I'm writing this, I can see that this has been covered before, and I might not have bothered if I'd known, but here it is anyway.
Mine is slightly different in that it uses the Custom HTML Prompt action and sets the System Clipboard to the result. The the prompt appears showing today's date and the next whole hour as the time by default.
The only reason I'm mentioning it is that people might come along here in the future and install this macro thinking that it's a general purpose cron generator and it is not.
I belive the correct term is "extended cron" which supports both "years" (up to 3000 AD) and "wildcards." I tried some of the "wildcards" that I found online, and the KM cron trigger supports it. Therefore KM is probably using "extended cron syntax" in its triggers.
I understand that nnCron has implemented a version that uses the year, but it's still not cron.
[very opinionated, very old unix sysadmin rant]
I'll be frank, I think it's just plain wrong of KM (sorry @peternlewis ) to call it a cron trigger and support the year field. cron has been around almost 50 years, it is what it is. Adding a year to it makes it something else. [/very opinionated, very old unix sysadmin rant]
You could be right. In which case, what word do you recommend KM use in place of "cron"?
Based on what I read online, the old cron is now called "standard cron" and the new improved cron is called "extended cron." Are you happy with those words?
Keyboard Maestro’s Cron trigger supports a variety of extensions to support real world macro needs like being able to trigger on the last Friday of the monty, and which may or may not match any specific implementation of any other version of cron.
If you wish to use only a strict specific version of cron from a specific unix system then you could spin one of those systems up and use the Remote trigger instead.