diff --git a/overviewer_core/tileset.py b/overviewer_core/tileset.py
index 3e13129..e665bff 100644
--- a/overviewer_core/tileset.py
+++ b/overviewer_core/tileset.py
@@ -24,6 +24,7 @@ import functools
 import time
 import errno
 import stat
+import platform
 from collections import namedtuple
 from itertools import product, izip, chain
 
@@ -760,8 +761,8 @@ class TileSet(object):
         #       Compare the last modified time of the chunk and tile. If the
         #       tile is older, mark it in a RendertileSet object as dirty.
 
-        for chunkx, chunkz, chunkmtime in self.regionset.iterate_chunks():
 
+        for chunkx, chunkz, chunkmtime in self.regionset.iterate_chunks() if (markall or platform.system() == 'Windows') else self.regionset.iterate_newer_chunks(last_rendertime): 
             chunkcount += 1
 
             if chunkmtime > max_chunk_mtime:
diff --git a/overviewer_core/world.py b/overviewer_core/world.py
index d958fd3..bbfc7cf 100644
--- a/overviewer_core/world.py
+++ b/overviewer_core/world.py
@@ -273,7 +273,7 @@ class RegionSet(object):
         
         for x, y, regionfile in self._iterate_regionfiles():
             # regionfile is a pathname
-            self.regionfiles[(x,y)] = regionfile
+            self.regionfiles[(x,y)] = (regionfile, os.path.getmtime(regionfile))
 
         self.empty_chunk = [None,None]
         logging.debug("Done scanning regions")
@@ -459,7 +459,7 @@ class RegionSet(object):
         
         """
 
-        for (regionx, regiony), regionfile in self.regionfiles.iteritems():
+        for (regionx, regiony), (regionfile, filemtime) in self.regionfiles.iteritems():
             try:
                 mcr = self._get_regionobj(regionfile)
             except nbt.CorruptRegionError:
@@ -468,6 +468,27 @@ class RegionSet(object):
             for chunkx, chunky in mcr.get_chunks():
                 yield chunkx+32*regionx, chunky+32*regiony, mcr.get_chunk_timestamp(chunkx, chunky)
 
+    def iterate_newer_chunks(self, mtime):
+        """Returns an iterator over all chunk metadata in this world. Iterates
+        over tuples of integers (x,z,mtime) for each chunk.  Other chunk data
+        is not returned here.
+        
+        """
+
+        for (regionx, regiony), (regionfile, filemtime) in self.regionfiles.iteritems():
+            """ SKIP LOADING A REGION WHICH HAS NOT BEEN MODIFIED! """
+            if (filemtime < mtime):
+                continue
+
+            try:
+                mcr = self._get_regionobj(regionfile)
+            except nbt.CorruptRegionError:
+                logging.warning("Found a corrupt region file at %s,%s. Skipping it.", regionx, regiony)
+                continue
+
+            for chunkx, chunky in mcr.get_chunks():
+                yield chunkx+32*regionx, chunky+32*regiony, mcr.get_chunk_timestamp(chunkx, chunky)
+
     def get_chunk_mtime(self, x, z):
         """Returns a chunk's mtime, or False if the chunk does not exist.  This
         is therefore a dual purpose method. It corrects for the given north
@@ -493,7 +514,7 @@ class RegionSet(object):
         Coords can be either be global chunk coords, or local to a region
 
         """
-        regionfile = self.regionfiles.get((chunkX//32, chunkY//32),None)
+        (regionfile,filemtime) = self.regionfiles.get((chunkX//32, chunkY//32),(None, None))
         return regionfile
             
     def _iterate_regionfiles(self):
@@ -537,6 +558,8 @@ class RegionSetWrapper(object):
         return self._r.get_chunk(x,z)
     def iterate_chunks(self):
         return self._r.iterate_chunks()
+    def iterate_newer_chunks(self,filemtime):
+        return self._r.iterate_newer_chunks(filemtime)
     def get_chunk_mtime(self, x, z):
         return self._r.get_chunk_mtime(x,z)
     
@@ -623,6 +646,11 @@ class RotatedRegionSet(RegionSetWrapper):
             x,z = self.rotate(x,z)
             yield x,z,mtime
 
+    def iterate_newer_chunks(self, filemtime):
+        for x,z,mtime in super(RotatedRegionSet, self).iterate_newer_chunks(filemtime):
+            x,z = self.rotate(x,z)
+            yield x,z,mtime
+
 class CroppedRegionSet(RegionSetWrapper):
     def __init__(self, rsetobj, xmin, zmin, xmax, zmax):
         super(CroppedRegionSet, self).__init__(rsetobj)
@@ -646,6 +674,14 @@ class CroppedRegionSet(RegionSetWrapper):
                     self.xmin <= x <= self.xmax and
                     self.zmin <= z <= self.zmax
                 )
+
+    def iterate_newer_chunks(self, filemtime):
+        return ((x,z,mtime) for (x,z,mtime) in super(CroppedRegionSet,self).iterate_newer_chunks(filemtime)
+                if
+                    self.xmin <= x <= self.xmax and
+                    self.zmin <= z <= self.zmax
+                )
+
     def get_chunk_mtime(self,x,z):
         if (
                 self.xmin <= x <= self.xmax and
diff --git a/test/test_tileset.py b/test/test_tileset.py
index f555eac..5658d00 100644
--- a/test/test_tileset.py
+++ b/test/test_tileset.py
@@ -53,6 +53,10 @@ class FakeRegionset(object):
         for (x,z),mtime in self.chunks.iteritems():
             yield x,z,mtime
 
+    def iterate_newer_chunks(self, filemtime):
+        for (x,z),mtime in self.chunks.iteritems():
+            yield x,z,mtime
+
     def get_chunk_mtime(self, x, z):
         try:
             return self.chunks[x,z]