diff --git a/src/calibre/devices/prs505/sony_cache.py b/src/calibre/devices/prs505/sony_cache.py
index 5f3e9118dc..b5a8f08541 100644
--- a/src/calibre/devices/prs505/sony_cache.py
+++ b/src/calibre/devices/prs505/sony_cache.py
@@ -354,19 +354,22 @@ class XMLCache(object):
             root = self.record_roots[i]
             lpath_map = self.build_lpath_map(root)
             gtz_count = ltz_count = 0
+            use_tz_var = False
             for book in booklist:
                 path = os.path.join(self.prefixes[i], *(book.lpath.split('/')))
                 record = lpath_map.get(book.lpath, None)
                 if record is None:
                     record = self.create_text_record(root, i, book.lpath)
-                (gtz_count, ltz_count) = self.update_text_record(record, book,
-                                                path, i, gtz_count, ltz_count)
+                (gtz_count, ltz_count, use_tz_var) = \
+                    self.update_text_record(record, book, path, i,
+                                            gtz_count, ltz_count, use_tz_var)
                 # Ensure the collections in the XML database are recorded for
                 # this book
                 if book.device_collections is None:
                     book.device_collections = []
                 book.device_collections = playlist_map.get(book.lpath, [])
-            debug_print('Timezone votes: %d GMT, %d LTZ'%(gtz_count, ltz_count))
+            debug_print('Timezone votes: %d GMT, %d LTZ, use_tz_var='%
+                                        (gtz_count, ltz_count, use_tz_var))
             self.update_playlists(i, root, booklist, collections_attributes)
         # Update the device collections because update playlist could have added
         # some new ones.
@@ -464,21 +467,27 @@ class XMLCache(object):
         root.append(ans)
         return ans
 
-    def update_text_record(self, record, book, path, bl_index, gtz_count, ltz_count):
+    def update_text_record(self, record, book, path, bl_index,
+                           gtz_count, ltz_count, use_tz_var):
         '''
         Update the Sony database from the book. This is done if the timestamp in
         the db differs from the timestamp on the file.
         '''
 
         # It seems that a Sony device can sometimes know what timezone it is in,
-        # and apparently converts the dates to GMT when it writes them to the
-        # db. Unfortunately, we can't tell when it does this, so we use a
-        # horrible heuristic. First, set dates only for new books, trying to
-        # avoid upsetting the sony. Use the timezone determined through the
-        # voting described next. Second, voting: if a book is not new, compare
-        # its Sony DB date against localtime and gmtime. Count the matches. When
-        # we must set a date, use the one with the most matches. Use localtime
-        # if the case of a tie, and hope it is right.
+        # and apparently converts the dates to GMT when it writes them to its
+        # DB. We can detect that a device is timezone-aware because there is a
+        # 'tz' variable in the Sony DB, which we can set to "0" to tell the
+        # device to ignore its own timezone when comparing mtime to the date in
+        # the DB.
+
+        # Unfortunately, if there is no tz variable in the DB, then we can't
+        # tell when the device applies a timezone conversion. We use a horrible
+        # heuristic to work around this problem. First, set dates only for new
+        # books, trying to avoid upsetting the sony. Second, voting: if a book
+        # is not new, compare its Sony DB date against localtime and gmtime.
+        # Count the matches. When we must set a date, use the one with the most
+        # matches. Use localtime if the case of a tie, and hope it is right.
         timestamp = os.path.getmtime(path)
         rec_date = record.get('date', None)
 
@@ -489,20 +498,25 @@ class XMLCache(object):
             return x
 
         if not getattr(book, '_new_book', False): # book is not new
-            if strftime(timestamp, zone=time.gmtime) == rec_date:
-                gtz_count += 1
-            elif strftime(timestamp, zone=time.localtime) == rec_date:
-                ltz_count += 1
+                if record.get('tz', None) is not None:
+                    use_tz_var = True
+                if strftime(timestamp, zone=time.gmtime) == rec_date:
+                    gtz_count += 1
+                elif strftime(timestamp, zone=time.localtime) == rec_date:
+                    ltz_count += 1
         else: # book is new. Set the time using the current votes
-            if ltz_count >= gtz_count:
+            if use_tz_var:
                 tz = time.localtime
-                debug_print("Using localtime TZ for new book", book.lpath)
+                record.set('tz', '0')
+                debug_print("Use localtime TZ and tz='0' for new book", book.lpath)
+            elif ltz_count >= gtz_count:
+                tz = time.localtime
+                debug_print("Use localtime TZ for new book", book.lpath)
             else:
                 tz = time.gmtime
-                debug_print("Using GMT TZ for new book", book.lpath)
+                debug_print("Use GMT TZ for new book", book.lpath)
             date = strftime(timestamp, zone=tz)
             record.set('date', clean(date))
-
         record.set('size', clean(str(os.stat(path).st_size)))
         title = book.title if book.title else _('Unknown')
         record.set('title', clean(title))
@@ -532,7 +546,7 @@ class XMLCache(object):
         if 'id' not in record.attrib:
             num = self.max_id(record.getroottree().getroot())
             record.set('id', str(num+1))
-        return (gtz_count, ltz_count)
+        return (gtz_count, ltz_count, use_tz_var)
     # }}}
 
     # Writing the XML files {{{