How to set lastmod to file modified date?

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I use the plugin last_modified. Its filename is jekyll-file-last-modified.rb, the file sits in the _plugins folder and thus does not need to be mentioned in _config.yml, it’s automatically available. Here’s the code:

# 2020-01-10 dealt with error when file does not exist
# 2019-01-18 added, and used in footer
# modified from (thank you!):
#   https://github.com/michaelx/jekyll-last-modified

module Jekyll
  class LastModifiedTag < Liquid::Tag

    def initialize(tag_name, path, tokens)
      super
      @path = path # copy into class property
    end

    def render(context)
      # Pipe parameter through Liquid to make additional replacements possible
      url = Liquid::Template.parse(@path).render context
	  
      # Adds the site source, so that it also works with a custom one
      site_source = context.registers[:site].config['source']
      file_path = site_source + '/' + url
      
      # ensure it works even if the file does not exist (yet), for tags
      begin
        File.mtime(file_path.strip!) # last modified date
      rescue
		"" # if exception, return empty string
	  end
    end

  end
end

Liquid::Template.register_tag('last_modified', Jekyll::LastModifiedTag)

To use this tag (in my footer include) I capture the file’s modification date like so:

{% capture lastModificationDate %}{% last_modified {{ page.path }} %}{% endcapture %}

This works fine for me. Hope this helps.