diff --git a/db_dtypes/__init__.py b/db_dtypes/__init__.py
index ca0b468..2b51bcd 100644
--- a/db_dtypes/__init__.py
+++ b/db_dtypes/__init__.py
@@ -150,7 +150,8 @@ def _datetime(
                 hour=int(hour),
                 minute=int(minute) if minute else 0,
                 second=int(second) if second else 0,
-                nanosecond=nanosecond,
+                microsecond=nanosecond // 1000,
+                nanosecond=nanosecond % 1000,
             ).to_datetime64()
         else:
             raise TypeError("Invalid value type", scalar)