This can be optionally supported. To be clear, does it mean almost "RANDOM at construction" and the parser will assign a value by a hash/random function and then the crontab struct has an immutable value of that field?

The way jenkins did it is they took the job name and hashed that. This gives you good staggering while still being consistent from run to run (compared to using a random number). This does mean it is immutable, not just at construction but ideally across separate constructions so long as the seed is the same.

For an API, I could see either taking in something that is hashable or just taking in a number and letting the caller choose their hashing. From the person's perspective writing cron syntax, they shouldn't care so long as they get the intended affect.