You could make use of Ecto’s virtual fields in order to add this key to your Schema struct and then have it encoded without creating a whole new Map.
Virtual fields on Ecto Schemas can be set and read but Ecto will not try to persist them to your database.
And since Ecto Schemas are really just normal Elixir structs, you can also simply update the value of your last_login field with the formatted string without it having any impact on your stored data.
