Changing month by +1 also adds a timestamp in the cell. How do I not insert the timestamp

Question

When I click the button "Change Dates" it updates the dates in the date column. It increases the dates by 1 month and inputs a Timestamp after the date. As you can see for the Date for Account 2 it increases the Date perfecting 1 month, but for Account 1 it increases it only 30 days from October 1st to October 31st.

I only want the month number to change and I don't want the timestamp inputted after the date.

function dateChanger() {
  var sheetName = "Sheet 1"; // Please set your sheet name.
  var a1Notations = ['G5:G27', 'T5:T27', 'AG5:AG27', 'AT5:AT27']; // This is from your question.

  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var ssId = ss.getId();
  var ranges = a1Notations.map(e => `'${sheetName}'!${e}`);
  var { valueRanges } = Sheets.Spreadsheets.Values.batchGet(ssId, { ranges, valueRenderOption: "UNFORMATTED_VALUE" });
  var data = valueRanges.map(({ values }, i) => {
    var values = values.map(([v]) => {
      var unixTime = (v - 25569) * 86400 * 1000; // Ref: https://stackoverflow.com/a/6154953
      var temp = new Date(unixTime);
      temp.setMonth(temp.getMonth() + 1);
      var serialNumber = (temp.getTime() / 1000 / 86400) + 25569; // Ref: https://stackoverflow.com/a/6154953
      return [serialNumber];
    });
    return { range: ranges[i], values };
  });
  Sheets.Spreadsheets.Values.batchUpdate({ data, "valueInputOption": "USER_ENTERED" }, ssId);
}

I've tried to get rid of the unixTime and Serial Number variable but it doesn't work.

Answer 1

From your provided Spreadsheet, the timezone offset might be related to your current issue. So, as one modification, how about the following modification?

From:

var data = valueRanges.map(({ values }, i) => {
  var values = values.map(([v]) => {
    var unixTime = (v - 25569) * 86400 * 1000; // Ref: https://stackoverflow.com/a/6154953
    var temp = new Date(unixTime);
    temp.setMonth(temp.getMonth() + 1);
    var serialNumber = (temp.getTime() / 1000 / 86400) + 25569; // Ref: https://stackoverflow.com/a/6154953
    return [serialNumber];
  });
  return { range: ranges[i], values };
});

To:

var data = valueRanges.map(({ values }, i) => {
  var values = values.map(([v]) => {
    var unixTime = (v - 25569) * 86400 * 1000; // Ref: https://stackoverflow.com/a/6154953
    var temp = new Date(unixTime);
    temp = new Date(unixTime + (temp.getTimezoneOffset() * 60 * 1000)); // Added
    temp.setMonth(temp.getMonth() + 1);
    var serialNumber = ((temp.getTime() - (temp.getTimezoneOffset() * 60 * 1000)) / 1000 / 86400) + 25569; // Modified Ref: https://stackoverflow.com/a/6154953
    return [serialNumber];
  });
  return { range: ranges[i], values };
});

• When I tested this modification using your provided Spreadsheet, it seems that 10/1/23 is changed to 11/1/23.