Populate one field from contents of another as it's being entered

Populate one field from contents of another as it's being entered

As the user types the "start year", I want to fill the "end year" with the same value, keystroke by keystroke. But simply using a keypress eventhandler doesn't do quite what I want, as

   $("#start_year").keypress(function(event) {
        $("#end_year").val($("#start_year").val());
    })

fills in "end_year" with the value before the keypress has been added to "start_year".

Again, I just want "end_year" to be the same as "start_year" as it's being typed.