foo.m

- (void)setPubDate:(NSString *)dateAsString {
  if(dateAsString == pubdate)
    return;
  [pubdate release];
  pubdate = [dateAsString copy];
  
  static NSDateFormatter *df = nil;
  if(!df) {
    df = [[NSDateFormatter alloc] init];
    df.locale = [[[NSLocale alloc] initWithLocaleIdentifier:@"en_US"] autorelease];
    df.timeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];
  }
  df.dateFormat = [self get_date_format];
  date_pubdate = [[df dateFromString:pubdate] retain];
}