diff --git a/django/utils/dateformat.py b/django/utils/dateformat.py
index f89300938e..0afda1850d 100644
--- a/django/utils/dateformat.py
+++ b/django/utils/dateformat.py
@@ -182,9 +182,11 @@ class DateFormat(TimeFormat):
         return MONTHS_AP[self.data.month]
 
     def O(self):
-        "Difference to Greenwich time in hours; e.g. '+0200'"
+        "Difference to Greenwich time in hours; e.g. '+0200', '-0430'"
         seconds = self.Z()
-        return u"%+03d%02d" % (seconds // 3600, (seconds // 60) % 60)
+        sign = '-' if seconds < 0 else '+'
+        seconds = abs(seconds)
+        return u"%s%02d%02d" % (sign, seconds // 3600, (seconds // 60) % 60)
 
     def r(self):
         "RFC 2822 formatted date; e.g. 'Thu, 21 Dec 2000 16:01:07 +0200'"
@@ -275,8 +277,10 @@ class DateFormat(TimeFormat):
         if not self.timezone:
             return 0
         offset = self.timezone.utcoffset(self.data)
-        # Only days can be negative, so negative offsets have days=-1 and
-        # seconds positive. Positive offsets have days=0
+        # `offset` is a datetime.timedelta. For negative values (to the west of
+        # UTC) only days can be negative (days=-1) and seconds are always
+        # positive. e.g. UTC-1 -> timedelta(days=-1, seconds=82800, microseconds=0)
+        # Positive offsets have days=0
         return offset.days * 86400 + offset.seconds
 
 def format(value, format_string):
diff --git a/django/utils/tzinfo.py b/django/utils/tzinfo.py
index fc7fa8250f..7ba3673df4 100644
--- a/django/utils/tzinfo.py
+++ b/django/utils/tzinfo.py
@@ -13,7 +13,7 @@ class FixedOffset(tzinfo):
         else:
             self.__offset = timedelta(minutes=offset)
 
-        sign = offset < 0 and '-' or '+'
+        sign = '-' if offset < 0 else '+'
         self.__name = u"%s%02d%02d" % (sign, abs(offset) / 60., abs(offset) % 60)
 
     def __repr__(self):
diff --git a/tests/regressiontests/utils/dateformat.py b/tests/regressiontests/utils/dateformat.py
index b678fa0093..f1ef720b20 100644
--- a/tests/regressiontests/utils/dateformat.py
+++ b/tests/regressiontests/utils/dateformat.py
@@ -127,3 +127,9 @@ class DateFormatTests(unittest.TestCase):
             self.assertEqual(dateformat.format(summertime, 'O'), u'+0200')
             self.assertEqual(dateformat.format(wintertime, 'I'), u'0')
             self.assertEqual(dateformat.format(wintertime, 'O'), u'+0100')
+
+        # Ticket #16924 -- We don't need timezone support to test this
+        # 3h30m to the west of UTC
+        tz = FixedOffset(-3*60 - 30)
+        dt = datetime(2009, 5, 16, 5, 30, 30, tzinfo=tz)
+        self.assertEqual(dateformat.format(dt, 'O'), u'-0330')