Calculate the difference of dates in days - date

Question

I need to calculate the difference in days between two dates in two differente occasions
Occasion 1 - There's a start and an end date
Occasion 2 - There's a start date and a "IN PROGRESS" where the end date should be
function myFunction() {
var ss = SpreadsheetApp.getActiveSpreadsheet();
var daTa = ss.getSheetByName("Data");
var daysColumn = daTa.getRange('C2:C' + daTa.getLastRow()).getValues();
var startDate = daTa.getRange('C2:C'+ daTa.getLastRow()).getValues().flat();
var endDate = daTa.getRange('A2:A'+ daTa.getLastRow()).getValues().flat();
var today = new Date().valueOf();
endDate.forEach((finaldate,row) => {
if(finaldate == "IN PROGRESS") {
daysColumn[row][0] = (parseInt(startDate,10)-today);
} else {
daysColumn[row][0] = (parseInt(startDate,10)-parseInt(finaldate,10));
}})
daTa.getRange(2,4,daysColumn.length, 1).setValues(daysColumn)
}
Right now i got this bit of code, and it know what needs to be done, but it returns only "#NUM!" values on the column, where it should print the numbers.

Answer 1

Probably you want this:
function myFunction() {
var ss = SpreadsheetApp.getActiveSpreadsheet();
var daTa = ss.getSheetByName("Data");
var daysColumn = daTa.getRange('C2:C' + daTa.getLastRow()).getValues();
var startDate = daTa.getRange('C2:C' + daTa.getLastRow()).getValues().flat()
.map(x => new Date(x).valueOf()); // get milliseconds of the dates
var endDate = daTa.getRange('A2:A' + daTa.getLastRow()).getValues().flat()
.map(x => new Date(x).valueOf()); // get milliseconds of the dates
var today = new Date().valueOf();
var day = 1000 * 60 * 60 * 24; // milliseconds in a day
endDate.forEach((finaldate, row) => {
if (finaldate == "IN PROGRESS") {
daysColumn[row][0] = (startDate[row] - today) / day;
} else {
daysColumn[row][0] = (startDate[row] - finaldate) / day;
}
})
daTa.getRange(2, 4, daysColumn.length, 1).setValues(daysColumn)
}

Answer 2

function DiffInDays(Day1,Day2) {
if(Day1 && Day2 && (Object.prototype.toString.call(Day1) === '[object Date]') && (Object.prototype.toString.call(Day2) === '[object Date]')) {
var day=86400000;
var t1=new Date(Day1).valueOf();
var t2=new Date(Day2).valueOf();
var d=Math.abs(t2-t1);
var days=Math.floor(d/day);
//Logger.log(days);
return days;
} else {
throw 'Invalid Inputs';
}
}