Use decimal rounding to fix high precision timedelta errors

src/marshmallow/fields.py

@@ -1471,7 +1471,9 @@ def _serialize(self, value, attr, obj, **kwargs):
         if value is None:
             return None
         base_unit = dt.timedelta(**{self.precision: 1})
-        return int(value.total_seconds() / base_unit.total_seconds())
+        delta = round(decimal.Decimal(value.total_seconds()), 9)
+        unit = round(decimal.Decimal(base_unit.total_seconds()), 9)
+        return int(delta / unit)
 
     def _deserialize(self, value, attr, data, **kwargs):
         try:

tests/test_serialization.py

@@ -712,6 +712,15 @@ def test_timedelta_field(self, user):
         user.d7 = None
         assert field.serialize("d7", user) is None
 
+        # https://github.com/marshmallow-code/marshmallow/issues/1856
+        user.d8 = dt.timedelta(milliseconds=345)
+        field = fields.TimeDelta(fields.TimeDelta.MILLISECONDS)
+        assert field.serialize("d8", user) == 345
+
+        user.d9 = dt.timedelta(milliseconds=1999)
+        field = fields.TimeDelta(fields.TimeDelta.SECONDS)
+        assert field.serialize("d9", user) == 1
+
     def test_datetime_list_field(self):
         obj = DateTimeList([dt.datetime.utcnow(), dt.datetime.now()])
         field = fields.List(fields.DateTime)