Source Wikipedia:
Question mark (?) is used instead of '*' for leaving either day-of-month or day-of-week blank.
The above statement is not making much sense to me.
So if I write some cron as 0 0 0 ? * * then does it mean first of every month or it means it will execute daily?
It is a bit confusing as Java crons start with seconds while other crons start with minute.