Find closest date in array with JavaScript

I have an array with days in it. Each day is an object, for example:

{day_year: "2012", day_month: "08", day_number: "03", day_name: "mon"}

I have also added a timestamp attribute to each day object, by using:

function convertDays() {
    var max_i = days.length;
    for(var i = 0; i < max_i; i++) {
        var tar_i = days[i];
        tar_i.timestamp = new Date(tar_i.day_year, tar_i.day_month, tar_i.day_number);
    }
}

The days in the array are arbitrary, so there is no real logic to them.

Now I want to find the two closest days to any given date. So if the array with days contains

• August 2, 2012
• August 4, 2012
• August 23, 2012

And I search for August 11, 2012, I want it to return August 4, 2012 and August 23, 2012.

I have tried using an answer from another question, that looks like this:

function findClosest(a, x) {
    var lo, hi;
    for(var i = a.length; i--;) {
        if(a[i] <= x && (lo === undefined || lo < a[i])) lo = a[i];
        if(a[i] >= x && (hi === undefined || hi > a[i])) hi = a[i];
    }
    return [lo, hi];
}

However, this returns unidentified.

What would be the most efficient (least processor/memory intensive way) to achieve this?

Edit: "However, how are those results "strange"? Could you provide an example of your code and data?"

I'm now using the following to generate an array of dates:

var full_day_array = [];
for(var i = 0; i < 10; i++) {
    var d = new Date();
    d.setDate(d.getDate() + i);
    full_day_array.push({day_year: d.getFullYear().toString(), day_month: (d.getMonth() + 1).toString(), day_number: d.getDate().toString()});
}

The strange part is, using the code below, this only works for an array of 10 dates or shorter. Whenever I use an array of 11