diff --git a/django/utils/dateformat.py b/django/utils/dateformat.py
index 6a91a370e5..b2586ba1ff 100644
--- a/django/utils/dateformat.py
+++ b/django/utils/dateformat.py
@@ -110,8 +110,8 @@ class TimeFormat(Formatter):
         return '%02d' % self.data.second
 
     def u(self):
-        "Microseconds"
-        return self.data.microsecond
+        "Microseconds; i.e. '000000' to '999999'"
+        return '%06d' %self.data.microsecond
 
 
 class DateFormat(TimeFormat):
diff --git a/docs/ref/templates/builtins.txt b/docs/ref/templates/builtins.txt
index 514953d666..07ac284905 100644
--- a/docs/ref/templates/builtins.txt
+++ b/docs/ref/templates/builtins.txt
@@ -1251,7 +1251,7 @@ S                 English ordinal suffix for day of the     ``'st'``, ``'nd'``,
                   month, 2 characters.
 t                 Number of days in the given month.        ``28`` to ``31``
 T                 Time zone of this machine.                ``'EST'``, ``'MDT'``
-u                 Microseconds.                             ``0`` to ``999999``
+u                 Microseconds.                             ``000000`` to ``999999``
 U                 Seconds since the Unix Epoch
                   (January 1 1970 00:00:00 UTC).
 w                 Day of the week, digits without           ``'0'`` (Sunday) to ``'6'`` (Saturday)
diff --git a/tests/regressiontests/utils/dateformat.py b/tests/regressiontests/utils/dateformat.py
index 0f18bb2a4d..0f4fd67f6f 100644
--- a/tests/regressiontests/utils/dateformat.py
+++ b/tests/regressiontests/utils/dateformat.py
@@ -72,6 +72,11 @@ class DateFormatTests(unittest.TestCase):
 
         self.assertEqual(dateformat.format(my_birthday, 'a'), 'p.m.')
 
+    def test_microsecond(self):
+        # Regression test for #18951
+        dt = datetime(2009, 5, 16, microsecond=123)
+        self.assertEqual(dateformat.format(dt, 'u'), '000123')
+
     def test_date_formats(self):
         my_birthday = datetime(1979, 7, 8, 22, 00)
         timestamp = datetime(2008, 5, 19, 11, 45, 23, 123456)