diff --git a/msgpack/ext.py b/msgpack/ext.py
index 9694819a..92ea4530 100644
--- a/msgpack/ext.py
+++ b/msgpack/ext.py
@@ -167,4 +167,4 @@ def from_datetime(dt):
 
         :rtype: Timestamp
         """
-        return Timestamp(seconds=int(dt.timestamp()), nanoseconds=dt.microsecond * 1000)
+        return Timestamp(seconds=int(dt.timestamp() // 1), nanoseconds=dt.microsecond * 1000)
diff --git a/test/test_timestamp.py b/test/test_timestamp.py
index 831141a1..6dfe3689 100644
--- a/test/test_timestamp.py
+++ b/test/test_timestamp.py
@@ -103,6 +103,13 @@ def test_timestamp_datetime():
 
     assert Timestamp.from_datetime(ts).to_datetime() == ts
 
+    # Regression test: pre-epoch fractional seconds must floor toward -inf.
+    pre_epoch = datetime.datetime(1969, 12, 31, 23, 59, 59, 500000, tzinfo=utc)
+    ts_pre_epoch = Timestamp.from_datetime(pre_epoch)
+    assert ts_pre_epoch.seconds == -1
+    assert ts_pre_epoch.nanoseconds == 500000000
+    assert ts_pre_epoch.to_datetime() == pre_epoch
+
 
 def test_unpack_datetime():
     t = Timestamp(42, 14)