environment: reduce calls to g_time_zone_new_local()

Creating a new GTimeZone for the local timezone can be quite expensive if
done repeatedly. It requires an open(), mmap(), and parsing of
/etc/localtime.

This patch was provided by Florian, and I've tested it as far back as
3.28.4 to ensure that we are really reducing the number of open() calls
on the compositor thread.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1051

Signed-off-by: Christian Hergert <chergert@redhat.com>

# Conflicts:
#	js/ui/environment.js

js/ui/environment.js

@@ -17,6 +17,9 @@ const Gtk = imports.gi.Gtk;
 const Lang = imports.lang;
 const Shell = imports.gi.Shell;
 const St = imports.gi.St;
+const System = imports.system;
+
+let _localTimeZone = null;
 
 // We can't import shell JS modules yet, because they may have
 // variable initializations, etc, that depend on init() already having
@@ -116,9 +119,26 @@ function init() {
         }
     };
 
+    // Override to clear our own timezone cache as well
+    const origClearDateCaches = System.clearDateCaches;
+    System.clearDateCaches = function () {
+        _localTimeZone = null;
+        origClearDateCaches();
+    };
+
     // Work around https://bugzilla.mozilla.org/show_bug.cgi?id=508783
     Date.prototype.toLocaleFormat = function(format) {
-        return Shell.util_format_date(format, this.getTime());
+        if (_localTimeZone === null)
+            _localTimeZone = GLib.TimeZone.new_local();
+
+        let dt = GLib.DateTime.new(_localTimeZone,
+            this.getYear(),
+            this.getMonth() + 1,
+            this.getDate(),
+            this.getHours(),
+            this.getMinutes(),
+            this.getSeconds());
+        return dt ? dt.format(format) : '';
     };
 
     let slowdownEnv = GLib.getenv('GNOME_SHELL_SLOWDOWN_FACTOR');