ios时间处理

1.获取时间的年,月,日,时,分,秒
  

 //"expireDatetime": "2015-07-28 17:29:48.0"   <==>   转换
    NSString *dateStr = [model.expireDatetime substringToIndex:model.expireDatetime.length-cutLength];
    //装换成日期
    NSDateFormatter *df = [[NSDateFormatter alloc]init];
    [df setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSDate *date = [df dateFromString:dateStr];
    //计算时间差
    //参数1:时间差里包含的内容(年,月,日,时,分,秒)
    //参数2:起始时间
    //参数3:结束时间
    //参数4:比较的一些附加参数
    NSCalendar *calendar = [NSCalendar currentCalendar];
    unsigned int unit = NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;
    NSDateComponents *dc = [calendar components:unit fromDate:[NSDate date] toDate:date options:0];