adding 7days to a date when copying from a google sheet to an other

here is a code I have done to copy and organise data from a spreadsheet to an other one.

function GENERAL() {
  var Cop = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("GO");
  var LP = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("0_Test-LISTE PROJETS");
  var values = LP.getRange('A4:V' + LP.getLastRow()).getValues();
  var res = values.flatMap(valueRange =>
    [
      [valueRange[5], valueRange[7], valueRange[9], valueRange[16], valueRange[20], 'duréé projet' /**color red*/],//project lenght
      [valueRange[5], valueRange[7], valueRange[9], valueRange[16], valueRange[17], 'phase de conception'/**color BLUE*/],//research lenght
      [valueRange[5], valueRange[7], valueRange[9], valueRange[17], valueRange[17]/**+ ' 7days'*/ , 'attente retour BAT'/**color white*/],//waiting for client
      [valueRange[5], valueRange[7], valueRange[9], valueRange[17] /**+ ' 7days'*/, valueRange[19], 'phase de fabrication'/**color green*/],//production time
      [valueRange[5], valueRange[7], valueRange[9], valueRange[19], valueRange[20], 'phase installation'/**color purple*/]//installation on site
    ]
  );
  Cop.getRange(2, 1, res.length, res[0].length).setValues(res);

}

as mentionned on the tittle, the problem is that I want to add 7 days to some of the date :
"valueRange[17]/**+ ' 7days'*"
But I just don't know how...
Thanks for your help !

I believe your goal is as follows.

• You want to add 7 days to the value of valueRange[17] in your script.
• I suppose that your value of valueRange[17] is the date object.

In this case, how about the following modification?

From:

var res = values.flatMap(valueRange =>
  [
    [valueRange[5], valueRange[7], valueRange[9], valueRange[16], valueRange[20], 'duréé projet' /**color red*/],//project lenght
    [valueRange[5], valueRange[7], valueRange[9], valueRange[16], valueRange[17], 'phase de conception'/**color BLUE*/],//research lenght
    [valueRange[5], valueRange[7], valueRange[9], valueRange[17], valueRange[17]/**+ ' 7days'*/ , 'attente retour BAT'/**color white*/],//waiting for client
    [valueRange[5], valueRange[7], valueRange[9], valueRange[17] /**+ ' 7days'*/, valueRange[19], 'phase de fabrication'/**color green*/],//production time
    [valueRange[5], valueRange[7], valueRange[9], valueRange[19], valueRange[20], 'phase installation'/**color purple*/]//installation on site
  ]
);

To:

var res = values.flatMap(valueRange => {
  valueRange[17] = new Date(valueRange[17].getTime() + (7 * 86400000));
  return [
    [valueRange[5], valueRange[7], valueRange[9], valueRange[16], valueRange[20], 'duréé projet' /**color red*/],//project lenght
    [valueRange[5], valueRange[7], valueRange[9], valueRange[16], valueRange[17], 'phase de conception'/**color BLUE*/],//research lenght
    [valueRange[5], valueRange[7], valueRange[9], valueRange[17], valueRange[17]/**+ ' 7days'*/, 'attente retour BAT'/**color white*/],//waiting for client
    [valueRange[5], valueRange[7], valueRange[9], valueRange[17] /**+ ' 7days'*/, valueRange[19], 'phase de fabrication'/**color green*/],//production time
    [valueRange[5], valueRange[7], valueRange[9], valueRange[19], valueRange[20], 'phase installation'/**color purple*/]//installation on site
  ];
});

Note:

• When an error like valueRange[17].getTime is not a function occurs, I think that the value of valueRange[17] is not the date object. In that case, can you provide the sample value of valueRange[17]?

Added:

From I still wanted to be able to use [17], in this case, how about the following modification? The converted value from valueRange[17] is put to temp.

To:

var res = values.flatMap(valueRange => {

  // var temp = new Date(valueRange[17].getTime() + (7 * 86400000));
  var temp = valueRange[17] instanceof Date ? new Date(valueRange[17].getTime() + (7 * 86400000)) : valueRange[17];

  return [
    [valueRange[5], valueRange[7], valueRange[9], valueRange[16], valueRange[20], 'duréé projet' /**color red*/],//project lenght
    [valueRange[5], valueRange[7], valueRange[9], valueRange[16], temp, 'phase de conception'/**color BLUE*/],//research lenght
    [valueRange[5], valueRange[7], valueRange[9], valueRange[17], temp/**+ ' 7days'*/, 'attente retour BAT'/**color white*/],//waiting for client
    [valueRange[5], valueRange[7], valueRange[9], temp /**+ ' 7days'*/, valueRange[19], 'phase de fabrication'/**color green*/],//production time
    [valueRange[5], valueRange[7], valueRange[9], valueRange[19], valueRange[20], 'phase installation'/**color purple*/]//installation on site
  ];
});

• About your new issue of valueRange[17].getTime is not a function, I have already mentioned it in my answer. For this, I updated the above script. Please confirm it.