DataMapper ORM v1.8.0

[eluser]WanWizard[/eluser]
Datamapper does relationships, to execute a union between two unrelated tables is not part of the functionality.

Either use a standard AR query, or retrieve both separately, create an array with the contents of both $object->all array's, and sort them in PHP.
If you use the date/time (with an id suffix to make sure it's unique) as the array key, and assign the all objects by reference, sorting will we quick and will not use extra memory.