Trying to add to dateTime in sheets

Question

function getFirstEmptyRow() {
  var spr = SpreadsheetApp.getActiveSpreadsheet();
  var column = spr.getRange('A:A');
  var values = column.getValues(); // get all data in one call
  var ct = 0;
  while ( values[ct][0] != "" ) {
    ct++;
  }
  return (ct);
}

function getLatestTime() {
  return SpreadsheetApp.getActiveSpreadsheet().getSheets()[0].getRange(getFirstEmptyRow(),1).getValue();
}

function getLatestPoints() {
  return SpreadsheetApp.getActiveSpreadsheet().getSheets()[0].getRange(getFirstEmptyRow(),2).getValue();
}

function getLatestAverage() {
  return SpreadsheetApp.getActiveSpreadsheet().getSheets()[0].getRange(getFirstEmptyRow(),3).getValue();
}

function daysLeft(){
  var pointsLeft = 24250 - getLatestPoints();
  return  (pointsLeft / getLatestAverage()) / 24;
}

function nextRedeem() { //Problem is with this function:
  var redeemTime = getLatestTime() + daysLeft() + (2/24); 
  return redeemTime;
}

In my sheet I have a list of rows with 1)a date/time 2)a point value and 3)A running average of points per hour. I am trying to write a function that figures out how much time is left before the points reach a certain number and add it to the latest time to figure out at what time I expect to have that number of points.

I have little experience with java script and weak typing. My problem is that when I try to add a number to my returned date value I either get a string or just NaN. My other problem is that sheets seems to interpret dates into a number differently than Number() does.

If my nextRedeem() function simply returns getLatestTime(), I can get sheets to show it either as a date or the number of days since 1/1/1900 or whatever it is. At that point, in a cell I can add to it. I can add getLatestTime() and daysLeft() in a cell and it works fine. I can also add the timezone offset and it works, in a cell. But when I do it in this function nothing seems to work. I have tried adding .value to the function calls and using parseFloat() but that gives me NaN.

How do I do arithmetic with these function returns?

Answer 1

The issue is that isn't how Dates are handled in javascript.

var redeemTime = getLatestTime().setDate( getLatestTime().getDate() + daysLeft() + 12 );

This will set the Day of the Month of your DateTime object to whatever the original date was + daysLeft(). I'm not sure what you're referring to with 2/24 (as that's always 12) but I included it in case you have some other context. If you add days over the number of days in the month, it will go into the next month, and similarly if you add negative days below 0, it will go into the previous month.

For more information on handling dates with JavaScript, see this link at W3Schools .

Answer 2

So Date.parse gets me half way there, but it give me milliseconds since 1/1/1970, where google is days since 12/30/1899. So I just had to use some math which I left uncomputed for clarity.

function dateToNum(date) {
  return (Date.parse(date)/1000/60/60/24 + 25569)
}

Now I can use dateToNum(getLatestTime()) and do whatever math I want to it. When this number is put into a cell that is formatted to datetime it will display correctly.