when do you call timerReset()? Perhaps you get that error when trying to call it after setTimeout() has already done its thing?
wrap it in
if (window.myTimeout) {
clearTimeout(myTimeout);
myTimeout = setTimeout("timerDone()", 1000 * 1440);
}
edit: Actually, upon further reflection, since you did mention jQuery (and yet don't have any actual jQuery code here... I wonder if you have this nested within some jQuery (like inside a $(document).ready(.. and this is a matter of variable scope. If so, try this:
window.message="Logged in";
window.myTimeout = setTimeout("timerDone()",1000 * 1440);
function timerDone()
{
window.message="Logged out";
}
function timerReset()
{
clearTimeout(window.myTimeout);
window.myTimeout = setTimeout("timerDone()", 1000 * 1440);
}