{% macro timestamp(time, is_relative=false, class_name=none) %}
  {# `datetime` value should be an ISO string #}
  <time
    class="timestamp {{ class_name if class_name }}" 
    datetime="{{ time }}"
  >
    {% if not caller %}
      {# this is going to be the formatted value according to `is_relative` #}
      {{ time if not is_relative else time}}
    {% else %}
      {{ caller() }}
    {% endif %}
  </time>
{% endmacro %}
