You could also refer to another cell containing the number of days back, eg =TODAY()-$Z$100. Then simply amend Z100 as necessary or leave it blank to keep to TODAY(). This method avoids your having to edit the cell containing the formula.
after you type the formula, you then have to click on the 'format' button, select the 'font' tab, and change select a font color of white before hitting the 'ok' button. do this and your conditional format should work as expected.