How to keep this javascript going instead of restarting on page onload?

[Mindi]

Hi, I have some javascript that rotates items. Problem is
as soon as you leave my web page and come back it starts over. Any way to keep it going all on its own so everything gets rotated?
Here's the code with the problem under the [2] content. Its set to change every hour, but starts over as soon as you revisit the page. Please let me know how to fix this so it doesn't start over when you visit the page but keeps going. That way if someone comes back to it is rotating something new. Thank you very much! Mindi

content[0] = ".txt";
content[1] = ".txt";
content[2] = ".txt";

var counter = 0;
function secondsplus_iframe()
{ document.getElementById("dynstuff").src = content[counter];
counter++; if (counter == content.length)
counter = 0; } setInterval("secondsplus_iframe()", 3600000);

[BurgerKing]

You will need to drop a cookie on the user's computer when you change ads, and then read it when the user visits the page again.

That way, you can start where you left off, rather than at the beginning.