mysqlの日時,date,datetime,timestampの違い。
DATE, DATETIME, TIMESTAMPの違い。
http://dev.mysql.com/doc/refman/5.5/en/datetime.html
The DATE type is used for values with a date part but no time part. MySQL retrieves and displays DATE values in ‘YYYY-MM-DD’ format. The supported range is ‘1000-01-01′ to ‘9999-12-31′.
The DATETIME type is used for values that contain both date and time parts. MySQL retrieves and displays DATETIME values in ‘YYYY-MM-DD HH:MM:SS’ format. The supported range is ‘1000-01-01 00:00:00′ to ‘9999-12-31 23:59:59′.
The TIMESTAMP data type is used for values that contain both date and time parts. TIMESTAMP has a range of ‘1970-01-01 00:00:01′ UTC to ‘2038-01-19 03:14:07′ UTC.
だそうです。
別の話ですが
ユニークID = timestamp + microsec + user_id
にします。