diff --git a/build.py b/build.py
index 142e5af..2dc06a4 100644
--- a/build.py
+++ b/build.py
@@ -14,6 +14,7 @@
 
 import argparse
 from bs4 import BeautifulSoup
+import datetime
 import glob
 import html
 from io import StringIO
@@ -169,11 +170,9 @@ def make_atom_feed():
   blog_entries.sort(key=operator.itemgetter('date'))
 
   entries_io = StringIO()
-  last_update = None
   for entry in blog_entries:
     # We lie and pretend that all entries were written at noon EST.
     update_date = entry['date'] + 'T12:00:00-04:00'
-    last_update = update_date
     entries_io.write(entry_template.format(
         url=entry['url'],
         title=entry['title'],
@@ -183,6 +182,8 @@ def make_atom_feed():
 
   entries_text = entries_io.getvalue()
 
+  now = datetime.datetime.now()
+  last_update = datetime.datetime.isoformat(now, 'T', 'seconds') + '-04:00'
   atom_feed = atom_template.format(
       last_update=last_update,
       entries=entries_io.getvalue())