added a comment - - edited
I think you need another type "Timestamp". Otherwise you cannot decide if it should be mapped to a "DATETIME" or "TIMESTAMP" column in mysql. A timestamp column only requires 4 bytes of storage space - datetime requires 8 bytes, so this is quite a decision regarding performance and storage space. a timestamp, of course, is limited to a date range. see http://dev.mysql.com/doc/refman/5.0/en/storage-requirements.html
I see that in D1 you mapped "timestamp" to "DATETIME". To be fully backwards compatible, a timstamp should be converted to a D2 "DateTime" type then (in this case ignore the first paragraph of this comment ).