My problem was that I was looking at it from the long angle:
function new_line() {
var html= '<div><br><input type="text" value="" id="dateP_'+ i +'"></div>';
document.getElementById("container").innerHTML += html;
$('#dateP_'+i).datepicker({
showOn: 'button',
buttonImage: 'calendar.gif',
buttonImageOnly: true
});
i++;
}