I don’t think it likes your created_at value:
blake@dev ~/code/discourse_api_sandbox (master) $ ruby new_topics.rb
/home/blake/.rvm/gems/ruby-2.4.2/gems/discourse_api-0.20.0/lib/discourse_api/client.rb:141:in `handle_error': ArgumentError at /posts (DiscourseApi::Error)
=======================
> argument out of range
lib/topic_creator.rb, line 126
------------------------------
``` ruby
121
122 @guardian.ensure_can_create!(Topic, category) unless (@opts[:skip_validations] || @opts[:archetype] == Archetype.private_message)
123
124 topic_params[:category_id] = category.id if category.present?
125
> 126 topic_params[:created_at] = Time.zone.parse(@opts[:created_at].to_s) if @opts[:created_at].present?
127
128 topic_params[:pinned_at] = Time.zone.parse(@opts[:pinned_at].to_s) if @opts[:pinned_at].present?
129 topic_params[:pinned_globally] = @opts[:pinned_globally] if @opts[:pinned_globally].present?
130
131 if SiteSetting.topic_featured_link_enabled && @opts[:featured_link].present? && @guardian.can_edit_featured_link?(topic_params[:category_id])
```