How should I store a date in PHP for use with javascript?

Question

What I want to do is make a script on the server read a text file, sort it, then output it to a javascript object (probably via JSON). The text file in question looks something like this:

13/09/2009,17/09/2009,Arbitrary dates
14/09/2009,18/09/2009,Some random comment
14/09/2010,18/12/2010,A comment to the dates
14/09/2010,18/09/2010,A subset of another date
14/09/2001,18/09/2002,The oldest date

The php to handle the filereading looks like this:

function loadDates()
{
    $dateFile = fopen("dates.txt", "rt");
    $dates = array();
    if($dateFile)
    {
        flock($dateFile,LOCK_SH);
        $i = 0;

        while(!feof($dateFile))
        {
            $text = fgets($dateFile);
            if($text !== FALSE)
            {
                $i++;
                $arr = explode(",",$text,3);
                //actual storage
                $dates[$i]['start']   = strtotime($arr[0]);
                $dates[$i]['end']     = strtotime($arr[1]);
                $dates[$i]['comment'] = $arr[2];
            }
        }
        fclose($dateFile);

        //sort by start date, then by end date
        foreach($dates as $key => $item)
        {
            $start[$key]  = $item['start'];
            $end[$key] = $item['end'];
        }
        array_multisort($start, SORT_ASC, $end, SORT_ASC, $dates);
        return $dates;
    }
    else
    {
        return FALSE;
    }
}

However, that stores unix timesstamps in the start and end dates. I would use the DateTime class, but I'm currently restricted to PHP 4.4. Ideally, I'd like to store the dates in a format that:

1. Can be compared numerically
2. Are human readable (allowing human editing of dates.txt)
3. Are consistently formatted (ie "01-01-1900" is converted to "01/01/1900")
4. Can be converted to a javascript Date object

How would I go about storing the dates so they satify these restrictions?

Answer 1

The safest is to use UNIX timestamps

in javascript, you can use

var mydate = new Date();
mydate.getTime(); //timestamp
mydate.setTime(your_timestamp); //set using timestamp

in php the date function takes the timestamp as second parameter.

see http://jp.php.net/manual/en/function.date.php and https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Objects/Date

EDIT:
Also see strftime http://jp.php.net/manual/en/function.strftime.php

EDIT:
Note: the javascript function takes milliseconds, and the php functions use seconds. divide the output of the javascript by 1000 or use something like the following:

Date.prototype.getTimeInSeconds = function() {
    return this.getTime()/1000;
}

var mydate = new Date();
mydate.getTimeInSeconds(); //PHP-compatible timestamp