Google Script - Running script by trigger changes date format

Question

I am new here, and somehow new for js for Google Scripts.

I have created a form, which fills a sheet, and want that a Calendar would be update with the data from the sheet. This form and calendar will be shared with a large group, so we all can create Events in a particular format, and to avoid deleting or modifying Events that should not be modified.

When I run the script from the workspace it runs properly, but after defining the trigger to run it, the date format changes (I believe that this is the reason, but I am not sure) and all Events are created for 1969. Below I present part of the script.

So, again, the script runs fine when I start it from the working space, but it changes the date format when running by trigger (onsubmit).
I have already removed and recreated the trigger, but it didn't work.
The date format is set as dd/MM/yyyy, and the time as HH:mm:ss.

Does anybody know how to fix this issue?
Thanks in advance
Best regards

function create_event_PRH(){\
  var calId = ID from Calendar;\
  var calendar = CalendarApp.getCalendarById(calId);\
  var timezone=calendar.getTimeZone();


  // Getting data from the spreadsheet\
  var ss = SpreadsheetApp.getActive();\
  var ss_name = ss.getName();\
  var sheet = ss.getActiveSheet();\
  var range = sheet.getDataRange();\
  var values = range.getValues();\
  var lastRow = sheet.getLastRow()-1;

  // Definition of the starting and finishing time in the spreadsheet (I am using this part of the script to sum date and time of the day)\
  var range = sheet.getRange('P'+String(lastRow+1)).setFormula('H'+String(lastRow+1)+'+I'+String(lastRow+1));\
  var range = sheet.getRange('Q'+String(lastRow+1)).setFormula('H'+String(lastRow+1)+'+J'+String(lastRow+1));\
  Logger.log(Utilities.formatDate(new Date(), timezone, 'dd/MM/yyyy HH:mm:ss'));

  // Start and end time
  var event_values = values[lastRow];\
  var event_start_time=new Date(event_values[15]);\
  var event_end_time=new Date(event_values[16]);


  // Event description and creation\
  var lecture_title=event_values[5];\
  var program_number=event_values[2];\
  var event_title = 'Program'+program_number+' - '+lecture_title;\
  var description='Presenter: '+event_values[10]+'\nInstitution:'+event_values[11];\
  var event = calendar.createEvent(event_title,event_start_time,event_end_time);\
  event = event.setDescription(description);\
}

Answer 1

specially @Jacques-GuzelHeron and @RobG,
The problem was related to the fact that I was using the spreadsheet to combine (add) date (dd/MM/yyyy) and time (HH:mm:ss) from the form, and retrieving it to get the date object. But I was not aware that the spreadsheet is not updated on the run, and the script uses only the cache data...
So, I had to combine both parameters manually. I had not found a better way to do this, but be code below is working fine.

  Logger.log(Utilities.formatDate(new Date(), 'America/Sao_Paulo', 'dd/MM/yyyy HH:mm:ss'));

  // Start and end time (the procedure below was needed to combine date and time)
  var event_values = values[lastRow];
  // The commands in the next lines were required to combine date and time
  // event_values[7] corresponds to the date of the event provided by the form to the spreadsheet as dd/MM/yyyy
  var day = new Date(event_values[7].toLocaleString()).getDate().valueOf();
  var month=new Date(event_values[7].toLocaleString()).getMonth().valueOf();
  var year=new Date(event_values[7].toLocaleString()).getFullYear().valueOf();
  // event_values[8] and [9] correspond to the start and end time of the event as HH:mm:ss also provided by the form to the spreadsheet
  var start_time=event_values[8].toString().split(":");
  // since the day was not provided for the start_time and end_time variables, in my case the script assumed as 1899. So I split the string first with ":" and then with "1899 ".
  var start_hour=start_time[0].split("1899 ")[1].valueOf();
  var start_min=start_time[1].valueOf();
  var end_time=event_values[9].toString().split(":")
  var end_hour=end_time[0].split("1899 ")[1].valueOf();
  var end_min=end_time[1].valueOf();

  var event_start_time = new Date(year,month,day,start_hour,start_min,"00");
  var event_end_time=new Date(year,month,day,end_hour,end_min,"00");