Date Arithmetic

I have a small but hard to solve problem to solve. Wrote a Select Query for my webpage. The query should select all values which where written into the DB the last seven Days. I thought everything was working fine, but now the month has changed and everything went wrong. My Select Query is not able to go into the Last month in calculation. I don't really know how to manage this problem. Please Help Here is the Query: $Query = "SELECT * FROM bild_db WHERE datum > '".date('Y') . '-' . date('m') . '-' . (date('d') - $_REQUEST['days']) . ' 00:00:00'."'"; Another code snipped: <? echo date('Y') . '-' . date('m') . '-' . (date('d') - 7); ?> The code will work fine as long you are not at the beginning of the month. Or in the first six days. Good luck Sascha