日期和字符串互转

日期和字符串
      //日期字符串转化为date
        NSDateFormatter*df = [[NSDateFormatter alloc]init];//格式化
        [df setDateFormat:@"yyyy-MM-dd"];
        self.date = [df dateFromString:self.dateStr];

//date转化为字符串
 NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
        [dateFormatter setDateFormat:@"yyyy-MM-dd"];
        NSString *strDate = [dateFormatter stringFromDate:self.date];