De acordo com a página do manual de crontab
(Seção 5):
Instead of the first five fields, one of eight special strings may appear:
string meaning ------ ------- @reboot Run once, at startup. @yearly Run once a year, "0 0 1 1 *". @annually (same as @yearly) @monthly Run once a month, "0 0 1 * *". @weekly Run once a week, "0 0 * * 0". @daily Run once a day, "0 0 * * *". @midnight (same as @daily) @hourly Run once an hour, "0 * * * *".
Please note that startup, as far as @reboot is concerned, is the time when the cron(8) daemon startup. In particular, it may be before some system daemons, or other facilities, were startup. This is due to the boot order sequence of the machine.
Execute man 5 crontab
no Debian Wheezy para ver a página inteira. ( Ou veja on-line )
Ele nunca foi removido, por isso também funciona em cada versão posterior.