def timestamp_to_ms(timestamp):
    """
    Converts a Unix timestamp into one with millisecond resolution.

    Parameters
    ----------
    timestamp : float or int
        Unix timestamp.

    Returns
    -------
    int
        `timestamp` with millisecond resolution (13 integer digits).

    """
    num_integer_digits = len(str(timestamp).split('.')[0])
    return int(timestamp*10**(13 - num_integer_digits))