Preserve modification timestamp when zipping files.

third_party/zlib/google/zip_reader.h

Index: third_party/zlib/google/zip_reader.h
diff --git a/third_party/zlib/google/zip_reader.h b/third_party/zlib/google/zip_reader.h
index a1f470421525197a68b04bc3bd10a2d8257f467c..a3e1ee82080c17a19351bd6c6b11e2d0710b4158 100644
--- a/third_party/zlib/google/zip_reader.h
+++ b/third_party/zlib/google/zip_reader.h
@@ -57,7 +57,15 @@ class ZipReader {
     // Returns 0 if the entry is a directory.
     int64 original_size() const { return original_size_; }
 
-    // Returns the last modified time.
+    // Returns the last modified time. If the time stored in the zip file was
+    // not valid, the unix epoch will be returned.
+    //
+    // The time stored in the zip archive uses the MS-DOS date and time format.
+    // http://msdn.microsoft.com/en-us/library/ms724247(v=vs.85).aspx
+    // As such the following limitations apply:
+    // * only years from 1980 to 2107 can be represented.
+    // * the time stamp has a 2 second resolution.
+    // * there's no timezone information, so the time is interpreted as local.
     base::Time last_modified() const { return last_modified_; }
 
     // Returns true if the entry is a directory.
@@ -127,7 +135,8 @@ class ZipReader {
   // instead. Returns true on success. OpenCurrentEntryInZip() must be
   // called beforehand.
   //
-  // This function does not preserve the timestamp of the original entry.
+  // This function preserves the timestamp of the original entry. If that
+  // timestamp is not valid, the timestamp will be set to the current time.
   bool ExtractCurrentEntryToFilePath(const base::FilePath& output_file_path);
 
   // Extracts the current entry to the given output directory path using
@@ -138,6 +147,9 @@ class ZipReader {
   //
   // Returns true on success. OpenCurrentEntryInZip() must be called
   // beforehand.
+  //
+  // This function preserves the timestamp of the original entry. If that
+  // timestamp is not valid, the timestamp will be set to the current time.
   bool ExtractCurrentEntryIntoDirectory(
       const base::FilePath& output_directory_path);