I installed it, used your example, changed a form value, pressed the "Send" button and "KeyError?: 'utc'" appears:
File ".../Trac-1.0-py2.7.egg-tmp/trac/ticket/templates/ticket_box.html", line 109, in <Expression u'wiki_to_html(context, ticket.description, escape_newlines=preserve_newlines)'>
${wiki_to_html(context, ticket.description, escape_newlines=preserve_newlines)}
File "build/bdist.win32/egg/trac/wiki/formatter.py", line 1546, in format_to_html
return HtmlFormatter(env, context, wikidom).generate(escape_newlines)
File "build/bdist.win32/egg/trac/wiki/formatter.py", line 1501, in generate
escape_newlines)
File "build/bdist.win32/egg/trac/wiki/formatter.py", line 1244, in format
self.handle_code_block(line, block_start_match)
File "build/bdist.win32/egg/trac/wiki/formatter.py", line 1134, in handle_code_block
processed = self.code_processor.process(code_text)
File "build/bdist.win32/egg/trac/wiki/formatter.py", line 356, in process
text = self.processor(text)
File "build/bdist.win32/egg/trac/wiki/formatter.py", line 343, in _macro_processor
text)
File "build/bdist.win32/egg/wikiforms/wikiforms.py", line 520, in expand_macro
time_string = format_datetime(last_modified,'%a %b %d %T %Y %Z',timezone(timezone_name))
File "build/bdist.win32/egg/trac/util/datefmt.py", line 874, in timezone
return _tzmap[tzname]