Google Sheets: Add a "last modified" timestamp to a row when a cell in a specific column is changed. Quick, hacky code, per usual.

ga_spreadsheet_onedit_timetimestamp.js

function onEdit( evt ) {
  var ss = SpreadsheetApp.getActiveSheet();
  var sheet_name = 'Bug List';

  if ( sheet_name !== ss.getName() ) {
    return;
  }

  var status_col = 8; // column number that contains the status type
  var lastmod_col = 10; // column number where the timestamp should go

  if ( status_col < evt.range.getColumn() || status_col > evt.range.getLastColumn() ) {
    return;
  }

  var ts = Utilities.formatDate( new Date(), 'America/Los_Angeles', 'yyyy-MM-dd HH:mm');
  var mod_cell, row, col, cell, cell_col;

  for ( row = 1; row <= evt.range.getNumRows(); row++ ) {
    for ( col = 1; col <= evt.range.getNumColumns(); col++ ) {
      cell = evt.range.getCell(row, col);
      cell.getColumn() = cell_col;
      if ( cell_col > status_col ) {
        continue;
      }

      if ( cell_col === status_col ) {
        mod_cell = ss.getRange( cell.getRow(), lastmod_col, 1, 1 );
        mod_cell.setNumberFormat('@').setValue( ts ); // '@' is necessary to force plain text
      }
    }
  }
  return;
}