dealing with dates on d3.js axis

You’re trying to use d3.scale.linear() for dates, and that won’t work. You need to use d3.time.scale() instead (docs):

// helper function
function getDate(d) {
    return new Date(d.jsonDate);
}

// get max and min dates - this assumes data is sorted
var minDate = getDate(data[0]),
    maxDate = getDate(data[data.length-1]);

var x = d3.time.scale().domain([minDate, maxDate]).range([0, w]);

Then you don’t need to deal with the time interval functions, you can just pass x a date:

.attr("d", d3.svg.line()
    .x(function(d) { return x(getDate(d)) })
    .y(function(d) { return y(d.jsonHitCount) })
);

Working fiddle here: http://jsfiddle.net/nrabinowitz/JTrnC/