Converts to float representing number of seconds since 1970-01-01 GMT.

def dt2ts(dt):
    """Converts to float representing number of seconds since 1970-01-01 GMT."""
    # Note: no assertion to really keep this fast
    assert isinstance(dt, (datetime.datetime, datetime.date))
    ret = time.mktime(dt.timetuple())
    if isinstance(dt, datetime.datetime):
        ret += 1e-6 * dt.microsecond
    return ret