cron

from cron(8):

       Additionally, cron checks each minute to see if its  spool  directory's

       modtime  (or  the  modtime on /etc/crontab) has changed, and if it has,

       cron will then examine the modtime on all  crontabs  and  reload  those

       which have changed.  Thus cron need not be restarted whenever a crontab

       file is modified.  Note that the crontab(1) command updates the modtime

       of the spool directory whenever it changes a crontab.