diff --git a/test/test_utils.py b/test/test_utils.py
index fceca03ec..c7e333522 100644
--- a/test/test_utils.py
+++ b/test/test_utils.py
@@ -421,6 +421,10 @@ def test_unified_timestamps(self):
         self.assertEqual(unified_timestamp('Wednesday 31 December 1969 18:01:26 MDT'), 86)
         self.assertEqual(unified_timestamp('12/31/1969 20:01:18 EDT', False), 78)
         self.assertEqual(unified_timestamp('2023-03-09T18:01:33.646Z', with_milliseconds=True), 1678384893.646)
+        # ISO8601 spec says that if no timezone is specified, we should use local timezone;
+        # but yt-dlp uses UTC to keep things consistent
+        self.assertEqual(unified_timestamp('2023-03-11T06:48:34.008'), 1678517314)
+
 
     def test_determine_ext(self):
         self.assertEqual(determine_ext('http://example.com/foo/bar.mp4/?download'), 'mp4')
diff --git a/yt_dlp/utils.py b/yt_dlp/utils.py
index bc2c36299..3edb7b053 100644
--- a/yt_dlp/utils.py
+++ b/yt_dlp/utils.py
@@ -1868,8 +1868,9 @@ def unified_timestamp(date_str, day_first=True, with_milliseconds=False):
 
     for expression in date_formats(day_first):
         with contextlib.suppress(ValueError):
-            dt = datetime.datetime.strptime(date_str, expression) - timezone + datetime.timedelta(hours=pm_delta)
-            return calendar.timegm(dt.timetuple()) + (dt.microsecond / 1e6 if with_milliseconds else 0)
+            dt = datetime.datetime.strptime(date_str, expression) + datetime.timedelta(hours=pm_delta)
+            dt_tz_aware = dt.replace(tzinfo=datetime.timezone(timezone))
+            return dt_tz_aware.timestamp() if with_milliseconds else int(dt_tz_aware.timestamp())
 
     timetuple = email.utils.parsedate_tz(date_str)
     if timetuple: