Issue #1: POSIXlt type is a struct, essentially. But because it is not a data.frame, we don't try to convert it to a struct. (We should probably convert named lists to structs and not list type in general.)
If I trick the converter into thinking it is a data.frame, it will convert to struct successfully.
Issue #2: round trip won't work because the attributes that tell you that this struct is a POSIXlt type, what time zone it is, etc., are dropped. This would be helped by storing those attributes as custom_metadata on the Table. (We could also implement it as an extension type, but if it's just for going back and forth between R, would that have any benefit?)