Adding Days to Date - Google Scripts

Question

I'm fairly new to this, so apologies if the answer is glaringly obvious.

I am trying to add and subtract days from today's date. The idea is eventually to calculate the date of next Monday and last Monday.

I've looked here which solved my first problem, but the next is perplexing me: subtracting days from today's date works without issue. Adding gives an invalid date.

The code is:

function findDate() {
   var d = new Date();
   var n = d.getDay();
   var makeDate = new Date(d.setDate(d.getDate()));
   Logger.log(makeDate)
   var weekDaysTo = new Array(7); //array of days to following Monday
   weekDaysTo[0]=  1
      weekDaysTo[1] = 7
      weekDaysTo[2] = 6
      weekDaysTo[3] = 5
      weekDaysTo[4] = 4
      weekDaysTo[5] = 3
      weekDaysTo[6] = 2

  Logger.log('weekDaysTo gives '+weekDaysTo[n])   

  var weekDaysFrom = new Array(7); //array of days to previous Monday
      weekDaysFrom[0]=  6
      weekDaysFrom[1] = 0
      weekDaysFrom[2] = 1
      weekDaysFrom[3] = 2
      weekDaysFrom[4] = 3
      weekDaysFrom[5] = 4
      weekDaysFrom[6] = 5

  var prevMon = new Date(makeDate+weekDaysTo[n]*3600000*24); //Converts ms     into days and adds

  Logger.log('Next Monday is '+prevMon);

  var followingMon = new Date(makeDate-weekDaysFrom[n]*3600000*24); //Converts ms into days
  Logger.log('Last Monday was ' +followingMon);

The Log Output is:

[16-05-22 21:17:58:419 ICT] Sun May 22 21:17:58 GMT+07:00 2016
[16-05-22 21:17:58:419 ICT] weekDaysTo gives 1
[16-05-22 21:17:58:420 ICT] Next Monday is Invalid Date 
[16-05-22 21:17:58:420 ICT] Last Monday was Mon May 16 2016 21:17:58 GMT+0700 (ICT)

This is irrespective of the value I add to the date. I can't see why subtracting works fine, but adding causes an invalid date. Changing the numbers added has no effect, changing it to subtraction works.

Answer 1

That's because JavaScript handles addition and subtraction different depending on the types of the operands.

> new Date()
Sun May 22 2016 14:45:00 GMT+0000 (UTC)
> new Date()+1
'Sun May 22 2016 14:45:05 GMT+0000 (UTC)1'
> new Date()-1
1463928307077

In your example, you should try converting makeDate to milliseconds before performing addition, either by doing +makeDate or makeDate.getTime() .

For example, instead of:

var prevMon = new Date(makeDate+weekDaysTo[n]*3600000*24);

Try:

var prevMon = new Date(+makeDate+weekDaysTo[n]*3600000*24);

Answer 2

That is extremely convoluted code for getting next Monday. Perhaps it works, but consider a far simpler algorithm. Hopefully the comments are sufficient:

 /* Given a date, return a new date for the following Monday. ** If date is a Monday, returns the following Monday. ** ** @param {Date} date - date to get next Monday of. ** - Default is current date. ** @returns {Date} - date for next Monday. */ function getNextMonday(date) { // Copy provided date or use current date date = date? new Date(+date) : new Date(); // Get the day number var dayNum = date.getDay(); // Set date to next Monday date.setDate(date.getDate() + (dayNum? 8 - dayNum: 1)); return date; } var x = new Date(2016,4,29) document.write('Start date: ' + x + '<br>Next Monday: ' + getNextMonday(x)); x = new Date(); document.write('<br><br>Start date: ' + x + '<br>Next Monday: ' + getNextMonday()); 

Here's a similar function for getting the previous Monday:

function getPreviousMonday(date) {
  date = date? new Date(+date) : new Date();
  var dayNum = date.getDay();
  // If Sunday, subtract 6. If Monday, subtract 7. Otherwise subtract dayNum - 1
  date.setDate(date.getDate() - (dayNum? (dayNum - 1 || 7) : 6));
  return date;
}