function UpdateDates() {
    var ss = SpreadsheetApp.getActiveSpreadsheet();
    var sheet = ss.getActiveSheet();
    var row = SpreadsheetApp.getActiveRange().getRow
    //Get the dates from the cell and convert them into Milliseconds since 1970/01/01
    var First = new Date(ss.getRange('F4').getValue()).getTime();
    var Second = new Date(ss.getRange('F5').getValue()).getTime()
    var Third = new Date(ss.getRange('F6').getValue()).getTime();
    var dayInMs = 24*60*60*1000    //one day in Milliseconds
    //add sevendays to each date in milliseconds

    First = First + (7*dayInMs)
    Second += (7*dayInMs)
    Third += (7*dayInMs)

   //Convert Milliseconds to date use new Date(time in ms) and set Values of the cell
    ss.getRange('F4').setValue(new Date(First));
    ss.getRange('F5').setValue(new Date(Second));
    ss.getRange('F6').setValue(new Date(Third));

}