Firstly "Date" is a reserved word (case sensitivity is dependant) so you should change the name of this column.
MySQL only uses 'YYYY-MM-DD' format for dates so your formats must match this (your $today).
http://dev.mysql.com/doc/refman/5.0/en/using-date.html
It may be better for you to use timestamps.
There are other ways of manipulating dates though: http://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html