code: function gpsTimestampToWnTow(gpsTimestamp) {
    var gpsTimeMs = gpsTimestamp.getTime() / 1000 - gpsEpochSeconds;
    var wn = Math.floor(gpsTimeMs / weekSeconds);
    var tow = gpsTimeMs - wn * weekSeconds;
    return { wn: wn, tow: tow };
  }
docs: Convert GPS Date timestamp (in GPS time, without leap seconds) to { wn, tow }.
  
  If you have a UTC timestamp, use `utcTimestampToWnTow`.
  
  @param {Date} gpsTimestamp - A timestamp representing a GPS timestamp, without leap-seconds.
  @return {object} { wn, tow }