$day = '2014-03-28';
$d1 = date("Y-m-d", strtotime("0 day $day"));//第1天03-28
$d3 = date("Y-m-d", strtotime("2 day $day"));//第3天
$d7 = date("Y-m-d", strtotime("6 day $day"));//第7天
$d15 = date("Y-m-d", strtotime("14 day $day"));//第15天
$d30 = date("Y-m-d", strtotime("29 day $day"));//第30天
$rd5 = date("Y-m-d", strtotime("-4 day $day"));//最后5天
$rd3 = date("Y-m-d", strtotime("-2 day $day"));//最后3天
$rd2 = date("Y-m-d", strtotime("-1 day $day"));//最后2天
$rd1 = date("Y-m-d", strtotime("-0 day $day"));//最后1天;