parseDate = d3.time.format("%Y") not working - d3.js

Question

My chart is working completely as expected, except that my parseDate function doesn't want to give me correct dates on the x-axis. I'm sure this is something simple.
Currently I'm adding d3.v2.min.js, without any additional helper libraries - do I need something else to get d3.time.format() working?
Without parsing the date my data returns an x-axis with:
.960 .965 .970 .975
rather than
1960 1965 1970 1975
The JSON is structured like this:
[{"year":1959,"average":315.97},
{"year":1960,"average":316.91},
{"year":1961,"average":317.64},
...etc
{"year":2011,"average":391.57}]
The code with the // commented sections being the issue:
<script>
var margin = {top: 20, right: 20, bottom: 30, left: 50},
width = 920 - margin.left - margin.right,
height = 500 - margin.top - margin.bottom;
// parseDate below not working
var parseDate = d3.time.format("%Y").parse;
var x = d3.time.scale()
.range([0, width]);
var y = d3.scale.linear()
.range([height, 0]);
var xAxis = d3.svg.axis()
.scale(x)
.orient("bottom");
var yAxis = d3.svg.axis()
.scale(y)
.orient("left");
// data taking format
d3.json("{{ asset('js/data_co2.json') }}", function(data) {
// parseDate() not working below
Gives error: Uncaught TypeError: Object 1959 has no method 'substring'
data.forEach(function(d) {
d.year = parseDate(d.year);
d.average = +d.average;
});
x.domain(d3.extent(data, function(d) { return d.year; }));
y.domain([260, 420]);
var area = d3.svg.area()
.x(function(d) { return x(d.year); })
.y0(height)
.y1(function(d) { return y(d.average); });
var svg = d3.select("#co2").append("svg")
.attr("width", width + margin.left + margin.right)
.attr("height", height + margin.top + margin.bottom)
.append("g")
.attr("transform", "translate(" + margin.left + "," + margin.top + ")");
svg.append("path")
.datum(data)
.attr("class", "area")
.attr("d", area);
svg.append("g")
.attr("class", "x axis")
.attr("transform", "translate(0," + height + ")")
.call(xAxis);
svg.append("g")
.attr("class", "y axis")
.call(yAxis)
.append("text")
.attr("transform", "rotate(-90)")
.attr("y", 6)
.attr("dy", ".71em")
.style("text-anchor", "end")
.text("CO2 Levels (ppm)");
});
</script>

Answer 1

parseDate, the function "prepared" by d3, expects to be passed a string for parsing. Your years are (appropriately) numbers, so you would need to convert them to strings:
parseDate(String(d.year));
or
parseDate(d.year.toString());
However, keep in mind that the same result – a Date object – could be achieved natively, like this:
var date = new Date(d.year, 0);// The required 0, is for January