hugo-book-theme/layouts/posts/single.html

{{ define "main" }}
{{- $dateFormat := default "January 2, 2006" .Site.Params.BookDateFormat -}}
<header>
  <h1>{{ .Title }}</h1>
  <h5>
    <strong>{{ .Date.Format $dateFormat }}</strong>
  </h5>
</header>
<article class="markdown">
  {{- .Content -}}
</article>
{{ end }}