How does entity attribute work?

[Cloud500]

I have an entity which contains a timestamp as the End time attribute.
If I read the documentation correctly, I can use the attribute parameter to display this as the status:

Entity object -> attribute (optional, added in 0.6.0)
Define an attribute to use instead of the state for the entity in the list

But basically, it doesn't matter what I write in attribute, it always shows last change...

Am I just too stupid or is this really a bug?
(I mean, I'm still quite new to the HA universe)

lovelace-home-feed-card version: 0.6.5

sensor attributes:

Message => str
All day => bool
Start time => timestamp
End time => timestamp
Description => str

card:

type: custom:home-feed-card
title: Current free Game
card_id: main_feed
show_empty: false
more_info_on_tap: true
state_color: true
compact_mode: true
entities:
  - entity: calendar.epic_games_store_rabatt_spiele
    content_template: "{{message}}"
    attribute: end_time

(I have tried various versions, end_time, “end_time”, “End time”, End time, {{end_time}}, and more, even message which works in content_template does not work in “attribute”.
However, if I enter "{{end_time}}" as content_template, the date is displayed.)

Any ideas?

PS:
I first tried it “normally” via calenders, but “last week” as status doesn't help me, because I want to know how much longer and not since when