What date format is this? Or is it even a date?
Those are dates stored as Unix timestamps, which are essentially the number of seconds since midnight, January 1st, 1970 UTC (the epoch is 1970-01-01T00:00:00+00:00).

As far as Drupal is concerned, they are proper fields on those entities, either TimestampItem, CreatedItem, or ChangedItem fields.

Keep in mind that the timestamp always refers to that epoch and always is referenced to UTC; there is no real thing as a Unix timestamp in a local time zone.  When working with them in Drupal it is always best to use DrupalDateTime objects, and use it methods for computation (which will inherit from \DateTime).