From 4035b7dd517f90ea7fcbbfa4bd71660793589cb5 Mon Sep 17 00:00:00 2001
From: Rio Yu <52408936+rioyu123@users.noreply.github.com>
Date: Sun, 4 Oct 2026 12:22:07 +0800
Subject: [PATCH] fix(google-api-core): preserve exact timestamp seconds

---
 .../google/api_core/datetime_helpers.py       |  2 +-
 .../tests/unit/test_datetime_helpers.py       | 43 +++++++++++++++++++
 2 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/packages/google-api-core/google/api_core/datetime_helpers.py b/packages/google-api-core/google/api_core/datetime_helpers.py
index 171a698d60c5..9ad5677311b7 100644
--- a/packages/google-api-core/google/api_core/datetime_helpers.py
+++ b/packages/google-api-core/google/api_core/datetime_helpers.py
@@ -268,7 +268,7 @@ def timestamp_pb(self):
             else self.replace(tzinfo=datetime.timezone.utc)
         )
         delta = inst - _UTC_EPOCH
-        seconds = int(delta.total_seconds())
+        seconds = delta.days * 86400 + delta.seconds
         nanos = self._nanosecond or self.microsecond * 1000
         return timestamp_pb2.Timestamp(seconds=seconds, nanos=nanos)
 
diff --git a/packages/google-api-core/tests/unit/test_datetime_helpers.py b/packages/google-api-core/tests/unit/test_datetime_helpers.py
index b8f614af74db..ebe4e0ee9db9 100644
--- a/packages/google-api-core/tests/unit/test_datetime_helpers.py
+++ b/packages/google-api-core/tests/unit/test_datetime_helpers.py
@@ -349,6 +349,49 @@ def test_timestamp_pb_w_nanos():
         )
         assert stamp.timestamp_pb() == timestamp
 
+    @staticmethod
+    @pytest.mark.parametrize(
+        "value",
+        [
+            "1969-12-31T23:59:59.123456789Z",
+            "1965-01-02T03:04:05.123456789Z",
+            "1969-12-31T23:59:59.000000001Z",
+            "1969-12-31T23:59:59Z",
+            "1970-01-01T00:00:00.123456789Z",
+            "2026-10-04T12:34:56.123456789Z",
+            "9999-12-31T23:59:59.999999999Z",
+        ],
+    )
+    def test_timestamp_pb_preserves_whole_seconds(value):
+        stamp = datetime_helpers.DatetimeWithNanoseconds.from_rfc3339(value)
+        expected = timestamp_pb2.Timestamp()
+        expected.FromJsonString(value)
+
+        assert stamp.timestamp_pb() == expected
+
+    @staticmethod
+    @pytest.mark.parametrize(
+        "tzinfo",
+        [
+            None,
+            datetime.timezone.utc,
+            datetime.timezone(datetime.timedelta(hours=5, minutes=30)),
+            datetime.timezone(datetime.timedelta(hours=-4)),
+        ],
+    )
+    @pytest.mark.parametrize(
+        "fraction", [{"microsecond": 123456}, {"nanosecond": 123456789}]
+    )
+    def test_timestamp_pb_fractional_seconds_with_timezone(tzinfo, fraction):
+        stamp = datetime_helpers.DatetimeWithNanoseconds(
+            1969, 12, 31, 23, 59, 59, tzinfo=tzinfo, **fraction
+        )
+        expected = timestamp_pb2.Timestamp()
+        expected.FromDatetime(stamp)
+        expected.nanos = fraction.get("nanosecond", stamp.microsecond * 1000)
+
+        assert stamp.timestamp_pb() == expected
+
     @staticmethod
     def test_from_timestamp_pb_wo_nanos():
         when = datetime.datetime(