Timestamps are in seconds. They're much more convenient to use, from my experience, then the inflexible SQL date format. Plus, they contain time, not merely the date.
So if you want days, take (60 x 60) x 24, giving you 86,400. 86,400 x 2, 3, ... 7 will give you those timestamps for that duration of days.
PHP Code:
// Place ranges into an array.
for ($i = 1; $i <= 7; $i++)
{
$array[] = TIMENOW - ($i * 86400);
}
Another method to get the timestamp (relative to today) without math is using strtotime():
PHP Code:
$seven_days_ago = strtotime('7 days ago');