Google Sheets Script Editor action not applying to all sheets

Question

I have a Sheets file with a tab/sheet for each month. I am editing the Script Editor to apply an automatic Timestamp when an item is scanned. How do I make the same script for each sheet, but make it that it only adds the timestamp to that specific sheet I'm working in and not all sheets?

Here's what I have. I have a file currently for each month, but when scanned in March, it goes to all months.

function onEdit(event)
{
  var timezone = "GMT-5";
  var timestamp_format = "MM-dd—yyyy"; // Timestamp Format.
  var updateColName = "ITEM";
  var timeStampColName = "DATE";
  var sheet = event.source.getSheetByName('MARCH'); //Name of the sheet where you want to run this script.  

  var actRng = event.source.getActiveRange();
  var editColumn = actRng.getColumn();
  var index = actRng.getRowIndex();
  var headers = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues();
  var dateCol = headers[0].indexOf(timeStampColName);
  var updateCol = headers[0].indexOf(updateColName); updateCol = updateCol+1;
  if (dateCol > -1 && index > 1 && editColumn == updateCol) { // only timestamp if 'Last Updated' header exists, but not in the header row itself! 
    var cell = sheet.getRange(index, dateCol + 1);
    var date = Utilities.formatDate(new Date(), timezone, timestamp_format);
    cell.setValue(date);
  }
}

Answer 1

Get the sheet that was edited. Change code to:

var sheet, ss;

ss = SpreadsheetApp.getActiveSpreadsheet();
sheet = ss.getActiveSheet();//Get the active sheet

Now the sheet variable is the active sheet that was just edited. So anything done with sheet will affect that sheet.