diff --git a/django/utils/feedgenerator.py b/django/utils/feedgenerator.py
index 7fc63f5adb..61d43b8136 100644
--- a/django/utils/feedgenerator.py
+++ b/django/utils/feedgenerator.py
@@ -53,17 +53,9 @@ def rfc2822_date(date):
 
 
 def rfc3339_date(date):
-    # Support datetime objects older than 1900
-    date = datetime_safe.new_datetime(date)
-    time_str = date.strftime('%Y-%m-%dT%H:%M:%S')
-    offset = date.utcoffset()
-    # Historically, this function assumes that naive datetimes are in UTC.
-    if offset is None:
-        return time_str + 'Z'
-    else:
-        timezone = (offset.days * 24 * 60) + (offset.seconds // 60)
-        hour, minute = divmod(timezone, 60)
-        return time_str + '%+03d:%02d' % (hour, minute)
+    if not isinstance(date, datetime.datetime):
+        date = datetime.datetime.combine(date, datetime.time())
+    return date.isoformat() + ('Z' if date.utcoffset() is None else '')
 
 
 def get_tag_uri(url, date):