Are you running the latest version (4.6). There was a bug report related to dates that may have also fixed this.
Here is a date column mapped using 4.6 and it works.
What you are seeing is the underlying numerical value that is stored for the date (the number of seconds since 1970). So the date is still stored but you are not seeing the appropriate format.
One work around (if you aren’t ready to upgrade) is use an expression column as a pass through to set the format to a date. Add an additional Expression column. Enter the name of the mapped column in the expression. Then expand the expression column options and set the type to a date.
Anyway, hopefully this is a bug that has been fixed but if you are running 4.6 now, please let us know! Thanks.