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)