The documentation states that for BigQuery data type DATE, protocol buffer types int32 and int64 are supported while the former is preferred. However, we found that int64 values are rejected by the Storage Write API because

1. In BQTableSchemaToProtoDescriptor#49, BigQuery data type DATE is mapped to protocol buffer type INT32.
2. In JsonToProtoMessage#272, Java data type Integer is expected for protocol buffer type INT32 and Java data type Long is ignored.

In addition, we could not understand why DATE fields do not support date literals, unlike DATETIME fields. In Standard SQL, both data types support date literals. You can also think the opposite way: Why does the Storage Write API not expect epoch microseconds for DATETIME fields while expecting epoch days for DATE fields?

Finally, JsonToProtoMessage does not convert Integer and Long into Double for DOUBLE fields although the conversion is absolutely safe.