Rename DateFormat to BookDateFormat, make format optional

layouts/posts/list.html

@@ -1,4 +1,5 @@
 {{ define "main" }}
+  {{- $dateFormat := default "January 2, 2006" .Site.Params.BookDateFormat -}}
   {{ $paginator := .Paginate (where .Pages "Params.hidden" "ne" true) }}
   {{ range sort .Paginator.Pages }}
   <article>
@@ -6,7 +7,7 @@
       <a href="{{ .RelPermalink }}">{{ .Title }}</a>
     </h2>
     <h5>
-      <strong>{{ .Date.Format .Site.Params.DateFormat }}</strong>
+      <strong>{{ .Date.Format $dateFormat }}</strong>
     </h5>
     <p class="markdown">
       {{- .Summary -}}

layouts/posts/single.html

@@ -1,8 +1,9 @@
 {{ define "main" }}
+{{- $dateFormat := default "January 2, 2006" .Site.Params.BookDateFormat -}}
 <header>
   <h1>{{ .Title }}</h1>
   <h5>
-    <strong>{{ .Date.Format .Site.Params.DateFormat  }}</strong>
+    <strong>{{ .Date.Format $dateFormat }}</strong>
   </h5>
 </header>
 <article class="markdown">