利用时间戳来准确计算某个时间点具现在的时间差

- (NSString *)intervalSinceNow: (NSString *) theDate 
{
    
    NSDateFormatter *date=[[NSDateFormatter alloc] init];
    [date setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSDate *d=[date dateFromString:theDate];
    
    NSTimeInterval late=[d timeIntervalSince1970]*1;
    
    
    NSDate* dat = [NSDate dateWithTimeIntervalSinceNow:0];
    NSTimeInterval now=[dat timeIntervalSince1970]*1;
    NSString *timeString=@"";
    
    NSTimeInterval cha=now-late;
    
    if (cha/3600<1) {
        timeString = [NSString stringWithFormat:@"%f", cha/60];
        timeString = [timeString substringToIndex:timeString.length-7];
        timeString=[NSString stringWithFormat:@"%@分钟前", timeString];
        
    }
    if (cha/3600>1&&cha/86400<1) {
        timeString = [NSString stringWithFormat:@"%f", cha/3600];
        timeString = [timeString substringToIndex:timeString.length-7];
        timeString=[NSString stringWithFormat:@"%@小时前", timeString];
    }
    if (cha/86400>1)
    {
        timeString = [NSString stringWithFormat:@"%f", cha/86400];
        timeString = [timeString substringToIndex:timeString.length-7];
        timeString=[NSString stringWithFormat:@"%@天前", timeString];
        
    }
    [date release];
    return timeString;
}