use last_modified_date page variable

add_timestamp.py

@@ -16,7 +16,7 @@ import subprocess
 def get_last_modified_date(markdown_file):
     '''Extract the date of the most recent commit
     that included changes to markdown_file.'''
-    git_log_cmd = "git log --pretty='format:%ai' {} | cut -d' ' -f1 | head -1"
+    git_log_cmd = "git log --pretty='format:%ai' {} | head -1"
     date_string = subprocess.check_output(git_log_cmd.format(markdown_file), shell=True).strip()
     return date_string
 
@@ -26,7 +26,7 @@ for filename in argv[1:]:
             lines = infh.readlines()
             assert lines[0].startswith('---'), "Error: {} doesn't appear to be a valid Markdown file with a YAML header. Please check you filenames and try again.".format(filename)
             last_modified = get_last_modified_date(filename)
-            lines.insert(1, "modified_date: '{}'\n".format(last_modified))
+            lines.insert(1, "last_modified_date: '{}'\n".format(last_modified))
             with open('{}.modified'.format(filename), 'w') as outfh:
                 for line in lines:
                     outfh.write(line)