How to format an integer field as Date/Time using Views module
Implement hook_entity_property_info_alter() to describe your custom entity type's properties to the Entity API module, which will then automatically create Views integration based on the types of fields in your database table.

http://www.drupalcontrib.org/api/drupal/contributions!entity!entity.api.php/function/hook_entity_property_info_alter/7

An example of this can be found in the Log module, which has three timestamps (timestamp, created, and changed). It gives each of these a "type" of "date" in the entity property info, which automatically makes them visible to Views as date fields. http://cgit.drupalcode.org/log/tree/log.module#n242