Google Sheets - Fill in date from another cell if empty - if-statement

Question

Thanks in advance for your help! Our team has a Google Sheet that is used to manage projects.
There is a Start Date (Column E) and a **Due Date **(Column F) that can be set for any task, however some require it, some don't. For example, for call tasks, we put the same start/end date. We need both cells to be filled in so we can display a gantt chart with all activities.
Is there a way to automatically fill in the Start Date cell with the End date (and vice versa), if one of them is empty?
I didn't want to populate those cells with a formula to keep things simple an played around with the formulas in conditional formatting but couldn't find a way!

Answer 1

This code is not very elegant (I'm still pretty new), but this may work if you are just looking to add a start date where there is an end date and vice versa.
EDIT: Using your implementation plan, I was able to get a version of this script to work. There was an issue where the startDate and endDate arrays held time data (ex: 4/21/2022, 12:00 PM). I was able to work around this by splitting the element from the comma so that it would only show the date.
function FillDate() {
let sheet = SpreadsheetApp.openById('YOUR_SHEET_ID').getSheetByName('YOUR_SHEET_NAME');
let dateColumns = sheet.getRange(13,4,sheet.getLastRow(),2).getValues();
let startDate = dateColumns.map(function(r){return r[0].toLocaleString();});
let endDate = dateColumns.map(function(r){return r[1].toLocaleString();});
let headerRows = 13; //you may want to change this if you have more than one header row
for (i=0; i<sheet.getMaxRows(); i++)
{
if (((startDate[i] != '') && (endDate[i] != ''))||((startDate[i] === '') && (endDate[i] === '')))
{
//This is empty because you don't want to do anything if both Start and End Dates are empty or full.
}
else
{
if((startDate[i] == '') && (endDate[i] != ''))
{
let splitEndDate = endDate[i].split(",");
sheet.getRange(headerRows+i,4).setValue(splitEndDate[0]);
}
else
{
let splitStartDate = startDate[i].split(",");
sheet.getRange(headerRows+i,5).setValue(splitStartDate[0]);
}
}
}
}