Hide some part of node when the field_countdown timer is zero
You might use twig without method. It will print all configured content to be rendered without the selected fields/properties.

{% set a = 'now'|date("U") %}
{% set b = node.field_timer.value|date("U") %}
{% if a &lt; b %}
    {# Show #}
    {{ content }}
{% else %}
    {# Hide - print the content without the field_timer #}
    {{ content|without('field_timer') }}
{% endif %}


For some cacheability issues it is recommended to always print content.