diff --git a/bson/objectid.py b/bson/objectid.py
index 35061b1..12facb0 100644
--- a/bson/objectid.py
+++ b/bson/objectid.py
@@ -184,7 +184,7 @@ def __generate(self):
         """
 
         # 4 bytes current time
-        oid = struct.pack(">i", int(time.time()))
+        oid = struct.pack(">i", int(time.time()) & 0xffffffff)
 
         # 3 bytes machine
         oid += ObjectId._machine_bytes