PHP: Date before X days

You can do that quite easy using the PHP function strtotime(). You can pass a date in text form to this function and you get a timestamp back.

You can even work with time constants such as day, week, month or year. Here are some examples for you:

$t = strtotime("-1 day");
echo date("d.m.Y", $t);

This outputs the date of yesterday, that is the date one day back in past.

$t = strtotime("-7 days");
echo date("d.m.Y", $t);

This outputs the date before one week, that is the date before seven days.

$t = strtotime("-10 days");
echo date("d.m.Y", $t);

Using this code, you get the date ten days ago.

$t = strtotime("-100 days");
echo date("d.m.Y", $t);

And using this code, you get the date one hundred days ago.
2016-11-01 at 23:52