diff --git a/overviewer_core/data/web_assets/overviewer.js b/overviewer_core/data/web_assets/overviewer.js
index 2ab35d0..8e3d471 100644
--- a/overviewer_core/data/web_assets/overviewer.js
+++ b/overviewer_core/data/web_assets/overviewer.js
@@ -1141,10 +1141,8 @@ var overviewer = {
                     }
                 }
                 url = url + '.' + pathExt;
-                if(overviewerConfig.map.cacheMinutes > 0) {
-                    var d = new Date();
-                    url += '?c=' + Math.floor(d.getTime() /
-                        (1000 * 60 * overviewerConfig.map.cacheMinutes));
+                if(typeof overviewerConfig.map.cacheTag !== 'undefined') {
+                    url += '?c=' + overviewerConfig.map.cacheTag;
                 }
                 return(urlBase + url);
             }
diff --git a/overviewer_core/data/web_assets/overviewerConfig.js b/overviewer_core/data/web_assets/overviewerConfig.js
index 5f3122f..5d0ae31 100644
--- a/overviewer_core/data/web_assets/overviewerConfig.js
+++ b/overviewer_core/data/web_assets/overviewerConfig.js
@@ -90,14 +90,14 @@ var overviewerConfig = {
          */
         'center':       {spawn_coords},
         /**
-         * Set this to tell browsers how long they should cache tiles in minutes.
-         * Essentially if set to 0, the url for tiles will end in .png
-         * if not set to 0 it will amend a number derived from the current time 
-         * to the end of the url, like .png?c=123456. This is a great method for 
-         * preventing browsers from caching the images. 0 saves bandwidth, not 0
-         * prevents caching.
+         * Overviewer will update cache_tag to a new value whenever the map has
+         * been updated. The tag will be added to the end of the tile URLs,
+         * like .png?c=123456. This is a great method for preventing browsers
+         * from loading old images from the cache when we know new versions are
+         * available. Use 'cacheTag': undefined to disable this behavior.
+         * undefined saves bandwidth, cache_tag ensures fresh image data.
          */
-        'cacheMinutes': 0,
+        'cacheTag':     {cache_tag},
         /**
          * Set to true to turn on debug mode, which adds a grid to the map along
          * with co-ordinates and a bunch of console output.
diff --git a/overviewer_core/googlemap.py b/overviewer_core/googlemap.py
index cbbce9e..2c0eb5e 100644
--- a/overviewer_core/googlemap.py
+++ b/overviewer_core/googlemap.py
@@ -19,7 +19,7 @@ import stat
 import cPickle
 import Image
 import shutil
-from time import strftime, localtime
+from time import strftime, time, localtime
 import json
 
 import util
@@ -128,6 +128,8 @@ class MapGen(object):
         config = config.replace("{spawn_coords}",
                                 json.dumps(list(self.world.spawn)))
 
+        config = config.replace("{cache_tag}", str(int(time())))
+
         #config = config.replace("{bg_color}", self.bg_color)
         
         # helper function to get a label for the given rendermode