user-facing support for BC book publication years

bookshare/trades/templates/detail_listing.html

@@ -84,6 +84,8 @@ SRCT Bookshare • {{ listing.title }}
   </div>
 {% endif %}
 
+{% load trades_extras %}
+
 <div class="row">
   <div class="col-xs-8">
     <h1>
@@ -106,7 +108,7 @@ SRCT Bookshare &bull; {{ listing.title }}
     <h4>
       <small>by</small> {{ listing.author }}
       {% if listing.year %}
-        ({{ listing.year }})
+        ({{ listing.year|bc }})
       {% endif %}
     </h4>
   </div>

bookshare/trades/templatetags/trades_extras.py

@@ -39,3 +39,11 @@ def empty_stars(avg_stars):
 @register.filter(name='int_maker')
 def int_maker(num):
     return int(num)
+
+@register.filter(name='bc')
+def bc(num):
+    if num < 0:
+        positive_value = str(abs(num))
+        return "%s B.C." % (positive_value)
+    else:
+        return num