We can only supply food for the people we need through a very complex manufacturing system. If we got EMP'd one day and no trucks could run, how long until you'd see widespread cannibalism? I predict less than a week in most places in the US.

@Top_failure: Someone forgets to maintain the walls keeping the Ocean from pouring into your city, and they break during a hurricane. The poor response time of the federal government leaves you and your family and possibly some neighbors stranded for a month (1 years supply for one person equals 1 month supply for 12).