Take a look at eZDateTimeType::title() method in the kernel/classes/datatypes/ezdatetime/ezdatetimetype.php file. I can suppose that reason behind of this change was that timestamp is not human readable as object name. Object name like 'Saturday 14 January 2006 10:32:00 pm' says more about the date. However you can always do sorting on datetime attribute using sort_by or play with output format using datetime operator.
I had the same problem and tried format the resulting object's name when using a ezdatetime datatype as object name pattern. I wanted something like YYYY-MM-DD, so that I could use the standard "sort by name"-function in the admin view to sort the items chronologically in the backend.
Because of Lukasz' tip I found out that eZ (I'm currently using version 3.6.1) uses the DateTimeFormat entry in the currently used locale ini-file (located in /share/locale) to create an object name based on an ezdatetime datatype.
In my case, I edited the file ger-DE.ini in /share/locale, where I changed the value DateTimeFormat=%l, %d.%F %Y %H:%i:%s in the DateTime settings group to DateTimeFormat=%Y-%m-%d %H:%i, which leads to a result like YYYY-MM-DD HH:MM.