Issue with date formatting and time zone change - datetime

Question

I am having a weird issue with dates formatting in Google Sheets and Google Script.
I am developing a script that reads a list of dates in a Google sheet and then sends automatic emails to a list of recipients. I have a cell in the spreadsheet with a date formatted as "dd/mm/yyyy" and then I use the following formulas (cell + 7, cell + 14, cell + 21, cell + 28, etc...) to generate all the other dates in the spreadsheet.
I use the following code to read the dates in the spreadsheet:
var sheet = SpreadsheetApp.getActiveSheet();
var drng = sheet.getDataRange();
var rng = sheet.getRange(4, 2, drng.getLastRow()-3, drng.getLastColumn());
var rngData = rng.getValues();
var todayDate = Utilities.formatDate(new Date(), "GMT+1", "dd/MM/yyyy");
// CYCLE THROUGH DATA
for (var i = 0; i < rngData.length; i++) { // cycle through the rows in the spreadsheet
var ptLastName = rngData[i][0]; // fixed patient data used to email customization
var ptFirstName = rngData[i][1];
var ptDoB = Utilities.formatDate(rngData[i][2], "GMT+1", "dd/MM/yyyy");
var ptSex = rngData[i][3];
var ptEmail = rngData[i][4];
var ptDoS = Utilities.formatDate(rngData[i][5], "GMT+1", "dd/MM/yyyy");
var ptDoD = Utilities.formatDate(rngData[i][6], "GMT+1", "dd/MM/yyyy");
for (var j = 7; j <= 14; j++) { // cycle through follow-up data for the current patient
var currentDate = Utilities.formatDate(rngData[i][j], "GMT+1", "dd/MM/yyyy");
if (currentDate == todayDate) { // send email with the questionnaire
writeEmail(ptFirstName, ptLastName, ptSex, ptDoD, ptEmail);
rng.getCell(i+1, j+1).setBackground("yellow");
}
}
}
The issue is that when I try for read and format some of the dates they are not corresponding to the ones listed in the spreadsheet. I have noted that the wrong dates in the code are systematically 1 day earlier than the ones listed in the spreadsheet. On a closer look I have noted that some of the dates read from the spreadsheet at GMT+2 while others are GMT+1.
That's weird because I have checked my system settings (it GMT+1) as well as the spreadsheet settings (GMT+1). So I don't understand why some dates are automatically converted to GMT+2.

Answer 1

I tried it this way and it works okay:
function dattest() {
const ss=SpreadsheetApp.getActive();
const sh=ss.getActiveSheet();
const rg=sh.getDataRange();
const vs=rg.getValues()[0];
vs.forEach(function(d){
Logger.log(Utilities.formatDate(new Date(d),Session.getScriptTimeZone(),"dd/MM/yyyy"));
});
}
Display Values:
8/1/2020,8/8/2020,8/15/2020,8/22/2020
Formulas:
,=A1+7,=A1+14,=a1+21
The output from view Executions:
Aug 1, 2020, 2:52:37 PM Info 01/08/2020
Aug 1, 2020, 2:52:37 PM Info 08/08/2020
Aug 1, 2020, 2:52:37 PM Info 15/08/2020
Aug 1, 2020, 2:52:37 PM Info 22/08/2020
Seems correct to me

Answer 2

thank you for your help.
I finally found out what the problem was.
The dates in the spreadsheet were stored as dd/mm/yyyy, and the system automatically added 00.00.00 as time to them. As a result, when I was trying to read some of the dates with the code some of them were converted as the day before because of the daylight saving time adjustment. I have managed to solve the issue adding 1/3 of a day to all the dates. This way they are stored as dd/mm/yyyy 8.00.000 am and they remain the same day all year long.