Subtract value at a set interval?
<p>I'd like to have a php function which will deduct a value, say, 50 from an initial integer of say, 500 at a set interval.</p>

<p>For example: Every 30 days, deduct 50 from 500. So after 90 days, you are left with 350, etc.</p>

<p>I can do subtractions, it's the interval routine that I can't figure out.</p>

<p>If not PHP, JS is great too. This is used to count down a value from the beginning of the year, to the end.</p>
