diff --git a/pungi/util.py b/pungi/util.py
index 6f456cb9..b6ecf6f3 100644
--- a/pungi/util.py
+++ b/pungi/util.py
@@ -807,7 +807,10 @@ def git_ls_remote(baseurl, ref):
 def get_tz_offset():
     """Return a string describing current local timezone offset."""
     is_dst = time.daylight and time.localtime().tm_isdst > 0
-    offset = time.altzone if is_dst else time.timezone
+    # We need to negate the value: the values are in seconds west of UTC, but
+    # ISO 8601 wants the offset to be negative for times behind UTC (i.e. to
+    # the west).
+    offset = -(time.altzone if is_dst else time.timezone)
     hours = offset / 3600
     minutes = (offset / 60) % 60
     return "%+03d:%02d" % (hours, minutes)
diff --git a/tests/test_util.py b/tests/test_util.py
index d40073fb..8911458b 100644
--- a/tests/test_util.py
+++ b/tests/test_util.py
@@ -597,18 +597,18 @@ class TestTZOffset(unittest.TestCase):
     @mock.patch('time.timezone', new=3600)
     @mock.patch('time.localtime', new=lambda: mock.Mock(tm_isdst=0))
     def test_zone_without_dst(self):
-        self.assertEqual(util.get_tz_offset(), "+01:00")
+        self.assertEqual(util.get_tz_offset(), "-01:00")
 
     @mock.patch('time.daylight', new=True)
     @mock.patch('time.altzone', new=7200)
     @mock.patch('time.timezone', new=3600)
     @mock.patch('time.localtime', new=lambda: mock.Mock(tm_isdst=0))
     def test_with_active_dst(self):
-        self.assertEqual(util.get_tz_offset(), "+01:00")
+        self.assertEqual(util.get_tz_offset(), "-01:00")
 
     @mock.patch('time.daylight', new=True)
-    @mock.patch('time.altzone', new=9000)
-    @mock.patch('time.timezone', new=3600)
+    @mock.patch('time.altzone', new=-9000)
+    @mock.patch('time.timezone', new=-3600)
     @mock.patch('time.localtime', new=lambda: mock.Mock(tm_isdst=1))
     def test_with_inactive_dst(self):
         self.assertEqual(util.get_tz_offset(), "+02:30")