Confirmed: Documentation (man page) indicates month range starts at 0. That not only conflicts with POSIX, but attempting such via crontab(1) results in it rejecting it as: 0: bad month Also confirmed likewise for day of month, documentation indicates starts at 0, that similarly fails, though the diagnostic differs a bit: bad day-of-month
So, at least as of the time of this writing, this currently impacts at least stable trixie/testing unstable (I didn't check oldstable nor earlier).
