Issue #3016427 by sysosmaster, nod_, kkalashnikov, Neslee Canil Pinto, alexpott, quicksketch: Default timezone selection incorrect

core/misc/timezone.es6.js

@@ -15,6 +15,14 @@
         .find('.timezone-detect')
         .once('timezone');
       if ($timezone.length) {
+        const tz = new Intl.DateTimeFormat().resolvedOptions().timeZone;
+        // Ensure that the timezone value returned by the browser is supported
+        // by the server.
+        if (tz && $timezone.find(`option[value="${tz}"]`).length) {
+          $timezone.val(tz);
+          return;
+        }
+
         const dateString = Date();
         // In some client environments, date strings include a time zone
         // abbreviation, between 3 and 5 letters enclosed in parentheses,

core/misc/timezone.js

@@ -11,6 +11,13 @@
       var $timezone = $(context).find('.timezone-detect').once('timezone');
 
       if ($timezone.length) {
+        var tz = new Intl.DateTimeFormat().resolvedOptions().timeZone;
+
+        if (tz && $timezone.find("option[value=\"".concat(tz, "\"]")).length) {
+          $timezone.val(tz);
+          return;
+        }
+
         var dateString = Date();
         var matches = dateString.match(/\(([A-Z]{3,5})\)/);
         var abbreviation = matches ? matches[1] : 0;