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?
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.