What date format is this? Or is it even a date? I'm working with a Drupal 8.3.6 setup done by someone else and I have a date format that doesn't make sense to me. I'm interacting with it thru MySQL Workbench.
The best I can tell,
1499287512 = Wed, 07/05/2017 - 23:02
and
1501079338 = Tue, 08/22/2017 - 06:25
BUT there's no guarantee of that.
In any event, 1499287512 and 1501079338 appear to be in a date placeholder; numbers like it appear in users_field_data.fields, created, changed, access, login.
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).