Google sheet script format changes before copy - google-sheets

Question

I have a script that will copy a line into another sheet when I enter a value in column 17 (the date when the file is closed).
I don't know why but before copying, the format changes and the date appears in numeric format
for example: 04/08/2017 => 42951
Any idea on what could be the problem?
Here is my script:
function onEdit(r) {
var sheet = r.source.getActiveSheet();
if(sheet.getName()!="Remboursements"){return}
var ss = SpreadsheetApp.getActiveSpreadsheet();
var source_sheet = ss.getSheetByName("Remboursements");
var target_sheet = ss.getSheetByName("Remboursements traités");
var range1 = r.range;
var columnOfCellEdited = range1.getColumn();
if (columnOfCellEdited === 17){
var source_range = source_sheet.getRange("A"+ range1.getRow()+":Q"+ range1.getRow());
var last_row = target_sheet.getLastRow();
target_sheet.insertRowAfter(last_row);
var target_range = target_sheet.getRange("A"+(last_row+1)+":Q"+(last_row+1));
source_range.copyTo(target_range);
source_sheet.deleteRow(range1.getRow())
}
}

Answer 1

Here's another way to do this:
function onEdit(r) {
if(r.source.getActiveSheet().getName()!="Remboursements"){return}
var source_sheet=r.source.getSheetByName("Remboursements");
var target_sheet=r.source.getSheetByName("Remboursements traités");
if (r.range.getColumn()==17)
{
var source_range=source_sheet.getRange("A"+ r.range.getRow() +":Q"+ r.range.getRow());
var values=source_range.getValues();
target_sheet.appendRow(values);
source_sheet.deleteRow(r.range.getRow())
}
}