Check out: http://www.php.net/manual/en/function.date.php
There is a section that tells you how to find dates in the future, or past.
$tomorrow = mktime (0,0,0,date("m") ,date("d")+1,date("Y"));
$lastmonth = mktime (0,0,0,date("m")-1,date("d"), date("Y"));
$nextyear = mktime (0,0,0,date("m"), date("d"), date("Y")+1);