Google script -- adding days to a date - google-apps-script

Question

I realize this is likely very easy but I'm a struggling newbie. I have dates in spreadsheet cells D3, D4, and D5 (mm/dd/yyyy) and simply need a macro to add 7 days to each of them. I've managed to mangle together some code that may be working but the output is in milliseconds rather than mm/dd/yyyy format.
I can't seem to get the syntax right to convert it. Any hints? Thank you!
function UpdateDates() {
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getActiveSheet();
var First = new Date();
var Second = new Date();
var Third = new Date();
var First = ss.getRange('D3').getValue();
var Second = ss.getRange('D4').getValue();
var Third = ss.getRange('D5').getValue();
ss.getRange('D3').setValue(First.getTime() + 7);
ss.getRange('D4').setValue(Second.getTime() + 7);
ss.getRange('D5').setValue(Third.getTime() + 7);
}

Answer 1

Adding days to date provides a unique challenge mainly because you have to take care of year, month and date. And increment each accordingly, hence just using getDate() doesn't work. You will need to keep track of the month and year. So as to change them if adding the seven days change the month or the year.
However, by using getTime() you can get the measure of time in milliseconds since Jan 1, 1970. And, add 7 days in milliseconds and convert it back to a date using new Date() constructor. Thus the constructor would take of figuring out the correct date based on the time.
Your code will look like so:
function UpdateDates() {
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getActiveSheet();
//Get the dates from the cell and convert them into Milliseconds since 1970/01/01
var First = new Date(ss.getRange('D3').getValue()).getTime();
var Second = new Date(ss.getRange('D4').getValue()).getTime()
var Third = new Date(ss.getRange('D5').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('D3').setValue(new Date(First));
ss.getRange('D4').setValue(new Date(Second));
ss.getRange('D5').setValue(new Date(Third));
}