diff --git a/pristine_zip/reference.py b/pristine_zip/reference.py
index 5060c71..2fce63b 100644
--- a/pristine_zip/reference.py
+++ b/pristine_zip/reference.py
@@ -1,44 +1,56 @@
 # Copyright (C) 2020 The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 """Creates a ZIP file deterministically. This can be used a base reference
 for delta files."""
 
 import os
 import subprocess
 
 
+def reset_timestamps(entries):
+    for entry in entries:
+        os.utime(entry, (0, 0))
+
+
 def walk(checkout_dir):
     entries = []
     for (dirpath, dirnames, filenames) in os.walk(checkout_dir):
         assert dirpath.startswith(checkout_dir)
         dirpath = dirpath[len(checkout_dir) :].lstrip("/")
         entries.append(dirpath)
-        entries.extend(os.path.join(dirpath, filename) for filename in filenames)
+        paths = [os.path.join(dirpath, filename) for filename in filenames]
+        entries.extend(paths)
     return entries
 
 
 def compress(checkout_dir: str, target: str):
     """Generates a reference zipball for the given checked out directory."""
     try:
         os.remove(target)
     except FileNotFoundError:
         pass
     assert os.path.isdir(checkout_dir), checkout_dir
 
+    entries_str = walk(checkout_dir)
+
+    reset_timestamps(os.path.join(checkout_dir, entry) for entry in entries_str)
+
     # Encode *before* sorting; sorting on unicode changes across configurations.
-    entries = [entry.encode() for entry in walk(checkout_dir)]
+    entries = [entry.encode() for entry in entries_str]
 
     # Sort entries ourselves; InfoZIP's zip does not guarantee order
     entries.sort()
 
     # -X = --no-extra, which prevents inclusion of extra non-deterministic
     # and implementation-dependant data
+    # -o = --latest-time, which sets the modification time of the zip to that
+    # of the most recent file
     proc = subprocess.run(
-        ["zip", "-X", target, "--names-stdin"],
+        ["zip", "-X", "-o", target, "--names-stdin"],
         cwd=checkout_dir,
         input=b"\n".join(entries),
     )
     proc.check_returncode()