adding days to historical dates in app script

Question

I am trying to pull some historical data, which has a variety of time stamps. I want the use to a user selected date, and then pull all days + some additional days. Date1 is

var Date1 = "Thu Oct 22 00:00:00 GMT-04:00 2020"
var startDate = new Date(Date1)

var Enddate = new Date();
Enddate.setDate(startDate + 10);

This doesn't work. I cant seem to figure out how to add the "10 days" to the Date1 variable.

"This doesn't work." What does that mean? Do you get an error? Is the date different from what you imagined it should be? — Heretic Monkey, Dec 10 '20 at 16:24
the log i get for Enddate is: Wed Dec 31 19:00:00 GMT-05:00 1969 , which is not correct. — Carlo B., Dec 10 '20 at 17:11

score 2 · Accepted Answer · answered Dec 10 '20 at 16:40

You need the getDate function of the Date object, and you'll need to pass the correct value when instantiating the new Date.

Try this:

var Date1 = "Thu Oct 22 00:00:00 GMT-04:00 2020"
var startDate = new Date(Date1);
// add it to startDate, using getDate()
startDate.setDate(startDate.getDate() + 10);

// now startDate will be 10 days later:
startDate; // Sun Nov 01 00:00:00 GMT-04:00 2020

// if you want an entirely new Date object, then instantiate a new one:
var Enddate = new Date(startDate);

If you want two different variables, then you can use the approach similar to what you tried, like so:

var Date1 = "Thu Oct 22 00:00:00 GMT-04:00 2020"
var startDate = new Date(Date1), second = new Date(Date1);      // we're using another temporary variable

// add the days
second.setDate(second.getDate() + 10);
// now use Enddate with this
var Enddate = new Date(second);

Enddate; // Sun Nov 01 00:00:00 GMT-04:00 2020

fantastic! this worked exactly as needed. Didn't realize I needed to use the getDate() after the variable — Carlo B., Dec 10 '20 at 17:19

terrymorse · Answer 2 · 2020-12-16T17:52:19.870

You can't just add a number to a Date object to get a new date.

You can add to a date by:

getting the current day of the month with getDate()
adding a number of days to that result
setting the new date with setDate()

const Date1 = "Thu Oct 22 00:00:00 GMT-04:00 2020";
const addDays = 10;

const startDate = new Date(Date1);
const startDays = startDate.getDate();

const newDate = new Date(startDate);
newDate.setDate(startDays + addDays);

console.log('startDate:', startDate.toISOString());
console.log('newDate:', newDate.toISOString());

Note:

Passing a string into the Date constructor is discouraged, as it produces inconsistent results across browsers.

It is better to pass in the individual date and time values:

const Date1 = new Date(2020, 9, 22, 0, 0, 0);

Adding a day by adding 24 hours isn't a good idea because where daylight saving is observed, not all days are 24 hours long. See the duplicate. — RobG, Dec 10 '20 at 23:19

adding days to historical dates in app script

2 Answers2