diff --git a/flask/helpers.py b/flask/helpers.py
index 37c458f5..4129ed30 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -483,8 +483,9 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
                           (default), this value is set by
                           :meth:`~Flask.get_send_file_max_age` of
                           :data:`~flask.current_app`.
-    :param last_modified: the Datetime object representing timestamp for when
-                          the input file was last modified.
+    :param last_modified: set the ``Last-Modified`` header to this value,
+        a :class:`~datetime.datetime` or timestamp.
+        If a file was passed, this overrides its mtime.
     """
     mtime = None
     if isinstance(filename_or_fp, string_types):
@@ -528,15 +529,10 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
     rv = current_app.response_class(data, mimetype=mimetype, headers=headers,
                                     direct_passthrough=True)
 
-    # if we know the file modification date, we can store it as
-    # the time of the last modification.
     if last_modified is not None:
-        if PY2:
-            rv.last_modified = int(last_modified.strftime("%s"))
-        else:
-            rv.last_modified = last_modified.timestamp()
+        rv.last_modified = last_modified
     elif mtime is not None:
-        rv.last_modified = int(mtime)
+        rv.last_modified = mtime
 
     rv.cache_control.public = True
     if cache_timeout is None:
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
index 9a07e0b9..3ff5900b 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -351,14 +351,15 @@ class TestSendfile(object):
 
     def test_send_file_last_modified(self):
         app = flask.Flask(__name__)
-        with app.test_request_context():
-            dtm = datetime.datetime.now()
-            rv = flask.send_file('static/index.html', last_modified=dtm)
-            if PY2:
-                assert rv.last_modified == int(dtm.strftime("%s"))
-            else:
-                assert rv.last_modified == dtm.timestamp()
-            rv.close()
+        last_modified = datetime.datetime(1999, 1, 1)
+
+        @app.route('/')
+        def index():
+            return flask.send_file(StringIO("party like it's"), last_modified=last_modified)
+
+        c = app.test_client()
+        rv = c.get('/')
+        assert rv.last_modified == last_modified
 
     def test_send_file_object(self):
         app = flask.Flask(__name__)