src/mongo/bson/bson_time_support.h: (pr comment)
They're only used in db/basic_types.idl as the (de)serializer for the millisEpoch etc type. But that means that they will be used by every generated struct that names those types. Currently there aren't any outside of unittest.idl. But the fact that it's in db/basic_types means it has to be as public as db/basic_types is. Potentially anyone can declare one as a struct member from any module and it has to work. There's an analogous situation for the unixEpoch type and its (de)serializer. The unixEpoch type does have two real uses in src/mongo/crypto/jwt_types.idl. It could be an action item here to remove millisEpoch, and move unixEpoch to the crypto idl file that uses it. And kill/move the (de)serializers accordingly.