// Takes current date and time, and makes it into the MySQL DATETIME column type format.
$now = date("Y-m-d H:i:s");
echo $now;
You can see this echos the current date and time in YYYY-MM-DD HH:MM:SS format (DATETIME). Once you have the variable, you just insert away. (I recommend you make it an INT type field, and store the datetime in UNIX TIMESTAMP format, which is like so:
// Takes current date and time, and makes it into a UNIX TIMESTAMP format.
$now = time();
echo $now;
It just makes it tons easier to manipulate it later...