Does cron have an authoritative standards document for cron syntax?

Question

Searching for examples of Cron usage and Cron parsing tools for cron's syntax, I see some tools with 5 input places and some with 6 (the sixth being for a seconds field). Furthermore, I see some flavors which support ? character as a valid input while other variants do not.

Does cron have an authoritative standards document for cron syntax? (E.g. an RFC or an ISO document). Or is cron just a loose collection of similar tools each with their own syntax rules?

Answer 1

I realise that this question is now over five years old. I stumbled upon it while searching for such a standard myself, and thought I would report my findings anyway.

The closest thing I could find to a standard definition of cron expression syntax is the IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008) for the crontab utility. Specifically, the section that defines the input files:

INPUT FILES

    In the POSIX locale, the user or application shall ensure that a crontab entry is a text file consisting of lines of six fields each. The fields shall be separated by <blank> characters. The first five fields shall be integer patterns that specify the following:

        Minute [0,59]

        Hour [0,23]

        Day of the month [1,31]

        Month of the year [1,12]

        Day of the week ([0,6] with 0=Sunday)