cron irregularity

There isn’t just one universal standard for cron expression syntax, as far as I can tell.

I noticed because the widely-used cron Rust crate (>2M downloads) that we use at Membrane includes a seconds field and handles day of week differently from the syntax I'm used to.

Or maybe there is an agreed-upon standard? The cron Wikipedia page has a section on "Nonstandard predefined scheduling definitions", which implies there are standard predefined scheduling definitions. And from the mcron man pages via gnu.org:

The format of a cron command is very much the V7 standard...The time and date fields are:

cron tl;dr

cron is a CLI and job scheduler for Unix-like OSs. crontab (cron table) files run shell commands on a schedule (colloquially, "cron jobs"). The cron daemon wakes up every minute, reads crontab files, and executes commands as appropriate.