To get a mysql formatted datetime using javascript:

**
 * You first need to create a formatting function to pad numbers to two digits…
 **/
function twoDigits(d) {
    if(0 <= d && d < 10) return "0" + d.toString();
    if(-10 < d && d < 0) return "-0" + (-1*d).toString();
    return d.toString();
}

/**
 * …and then create the method to output the date string as desired.
 * Some people hate using prototypes this way, but if you are going
 * to apply this to more than one Date object, having it as a prototype
 * makes sense.
 **/
Date.prototype.toMysqlFormat = function() {
    return this.getUTCFullYear() + "-" + twoDigits(1 + this.getUTCMonth()) + "-" + twoDigits(this.getUTCDate()) + " " + twoDigits(this.getUTCHours()) + ":" + twoDigits(this.getUTCMinutes()) + ":" + twoDigits(this.getUTCSeconds());
};

I couldn't find a simpler jQuery solution to this.
If you want the time according to the user's time zone, remove the "UTC" in the second last line of the code.