iOS/OC: 按时间戳获得该天零点对应的时间戳

为了按实现根据时间戳计算日期并按日期分组,写了个NSDate的分类:
--按时间戳获得该天零点对应的时间戳--
如下:

#import "NSDate+JLZero.h"

@implementation NSDate (JLZero)

+ (double)getZeroWithTimeInterverl:(NSTimeInterval) timeInterval
{
    NSDate *originalDate = [NSDate dateWithTimeIntervalSince1970:timeInterval];
    NSDateFormatter *dateFomater = [[NSDateFormatter alloc]init];
    dateFomater.dateFormat = @"yyyy年MM月dd日";
    NSString *original = [dateFomater stringFromDate:originalDate];
    NSDate *ZeroDate = [dateFomater dateFromString:original];
    return [ZeroDate timeIntervalSince1970];
}

恩,看上去有点诡异...