So, you want a Google App Script function that will allow you to insert the current date time stamp, but then that date time stamp should not ever change again?

start.stop.function.js

/**
* This is your `=START()` function.
* It will make a call to the `handleTimestamp_(propertyName)`
* Which does the real work.
*/
function START(){
  SpreadsheetApp.getActiveRange().getA1Notation()
  return handleTimestamp_('starttimestamep'.concat(SpreadsheetApp.getActiveRange().getA1Notation()))
}

/**
* This is your `=END()` function.
* It will make a call to the `handleTimestamp_(propertyName)`
* Which does the real work.
*/
function END(){
  return handleTimestamp_('endtimestamep'.concat(SpreadsheetApp.getActiveRange().getA1Notation()))
}

function RESET_START(){
  SpreadsheetApp.getActiveRange().getA1Notation()
  PropertiesService.getScriptProperties().deleteProperty('starttimestamep'.concat(SpreadsheetApp.getActiveRange().getA1Notation()))
}

function RESET_END(){
  SpreadsheetApp.getActiveRange().getA1Notation()
  PropertiesService.getScriptProperties().deleteProperty('endtimestamep'.concat(SpreadsheetApp.getActiveRange().getA1Notation()))
}

/**
* This is where the real work begins. Notice first that we pass
* in the property name that we are going to work with.
*/
function handleTimestamp_(propertyName)
{
  // First, we check to see if the property exists. You can think
  // of properties like global variables. They will sorta be there
  // as long as your spreadsheet is. Simply, behinds the scenes where
  // no one can see them.
  if (! PropertiesService.getScriptProperties().getProperty(propertyName)){
    // Well, if we got here, the property doesn't exist.
    
    var timestamp = new Date()
    
    // Easy money, we set the property to the current Date/Time
    PropertiesService.getScriptProperties().setProperty(propertyName, timestamp)
  }
  
  // Regardless of what happened above, we now get the property, which was either
  // just created, or was created a long time ago, and return that.
  return new Date(PropertiesService.getScriptProperties().getProperty(propertyName))
}