def express_as_duration(delta)
    timestamp = []
    hours = int(delta / (60 * 60))
    minutes = int((delta % (60 * 60)) / 60)
    seconds = delta % 60

    if hours > 0:
        if hours == 1:
            timestamp.append("1 hour")
        else:
            timestamp.append("{} hours".format(hours))
    if minutes > 0:
        if minutes == 1:
            timestamp.append("1 minute")
        else:
            timestamp.append("{} minutes".format(minutes))
    timestamp.append("{0:.2f} seconds".format(seconds))
    return ', '.join(timestamp)