constant

constant returns the constant value for a given string:

{{ some_date|date(constant('DATE_W3C')) }}
{{ constant('Namespace\\Classname::CONSTANT_NAME') }}

As of 1.12.1 you can read constants from object instances as well:

{{ constant('RSS', date) }}

Use the defined test to check if a constant is defined:

{% if constant('SOME_CONST') is defined %}
    ...
{% endif %}