Simple date diff in JavaScript. The ones I found out there were using division and modulus and would not represent 28 days between Feb and March as 1 month. None of them took into account daylight savings either.

Date#diff.js

/**
 * Returns an object with properties representing the number of years, months, days, hours, minutes
 * and seconds between the `this` date and a passed date or Date.now
 */
Object.defineProperty(Date.prototype, 'diff', {
    writable: true,
    configurable: true,
    value: function (/*[ instanceof Date || instanceof Number ]*/) {
        var diff = new Date(),
            op   = arguments[0] == null ? new Date() : new Date(arguments[0]),
            abs  = !!arguments[typeof arguments[1] === 'undefined' ? 0 : 1];

        if (isNaN(op) || isNaN(this))
            throw new Error ('Tried to diff an invalid date.');

        var p  = this > op,
            d1 = p ? this : op,
            d2 = p ? op : this,
            y = d1.getUTCFullYear() - d2.getUTCFullYear(),
            m = d1.getUTCMonth()    - d2.getUTCMonth(),
            d = d1.getUTCDate()     - d2.getUTCDate(),
            h = d1.getUTCHours()    - d2.getUTCHours(),
            i = d1.getUTCMinutes()  - d2.getUTCMinutes(),
            s = d1.getUTCSeconds()  - d2.getUTCSeconds();

        if (s < 0)
            s = 60 + s,
            i--;

        if (i < 0)
            i = 60 + i,
            h--;

        if (h < 0)
            h = 24 + h,
            d--;

        if (d < 0)
            d = (new Date(d1.getUTCFullYear(), d1.getUTCMonth(), 0).getUTCDate()) + d,
            m--;

        if (m < 0)
            m = 12 + m,
            y--;

        return {
            isPast: p,
            years:  y,
            months: m,
            days:   d,
            hours:  h,
            mins:   i,
            secs:   s
        };
    }
});