#define kDEFAULT_DATE_TIME_FORMAT (@"yyyy-MM-dd HH:mm:ss")

//获取当前日期，时间
+(NSDate *)getCurrentDate{
    NSDate *now = [NSDate date];
    return now;
}

//将日期转换为字符串（日期，时间）
+(NSString *)getDateStringFromDate:(NSDate *)date{
    NSInteger location = 0;
    NSString *timeStr = @"";
    NSDateFormatter *formatter = [[[NSDateFormatter alloc] init] autorelease];
    [formatter setLocale:[[[NSLocale alloc] initWithLocaleIdentifier:@"en_US"] autorelease]];
    [formatter setDateFormat:@"HH:mm:a"];
    NSString *ampm = [[[formatter stringFromDate:date] componentsSeparatedByString:@":"] objectAtIndex:2];
    timeStr = [formatter stringFromDate:date];
    NSRange range = [timeStr rangeOfString:[NSString stringWithFormat:@":%@",ampm]];
    location = range.location;
    NSString *string = [timeStr substringToIndex:location];
    timeStr = [NSString stringWithFormat:@"%@ %@",ampm,string];
    
    
    NSString *dateStr = @"";
    NSDateFormatter *Dformatter = [[[NSDateFormatter alloc] init] autorelease];
    [Dformatter setLocale:[[[NSLocale alloc] initWithLocaleIdentifier:@"en_US"] autorelease]];
    [Dformatter setDateFormat:@"yyyy-MM-dd"];    
    dateStr = [Dformatter stringFromDate:date];
//    NSLog(@"%@", [NSString stringWithFormat:@"%@  %@",dateStr,timeStr]);
    return [NSString stringWithFormat:@"%@  %@",dateStr,timeStr];

}

//计算两个日期之间的差距，过了多少天。。
+(NSInteger)getDateToDateDays:(NSDate *)date withSaveDate:(NSDate *)saveDate{
    NSCalendar* chineseClendar = [ [ NSCalendar alloc ] initWithCalendarIdentifier:NSGregorianCalendar ];  
    NSUInteger unitFlags =  NSHourCalendarUnit | NSMinuteCalendarUnit |  
    NSSecondCalendarUnit | NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit;  
    NSDateComponents *cps = [ chineseClendar components:unitFlags fromDate:date  toDate:saveDate  options:0];  
    NSInteger diffDay   = [ cps day ];  
    [chineseClendar release];
    return diffDay;
}


例：NSDate *lastDate = [self getSaveDate];//saveDate通过将NSDate转换为NSString来保存
    currentDate = [NSDate date];
    NSInteger day = [DateHelper getDateToDateDays:currentDate withSaveDate: lastDate];

//日期转字符串
+ (NSString * )NSDateToNSString: (NSDate * )date
{    
    NSDateFormatter * formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat: kDEFAULT_DATE_TIME_FORMAT];
    NSString *dateString = [formatter stringFromDate:date];
    [formatter release];
    return dateString;
}

//字符串转日期
+ (NSDate * )NSStringToNSDate: (NSString * )string
{    
    NSDateFormatter * formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat: kDEFAULT_DATE_TIME_FORMAT];
    NSDate *date = [formatter dateFromString :string];
    [formatter release];
    return date;
}

//1970年到现在的秒数转换成时间显示

- (NSString *)bySecondGetDate:(NSString *)second{
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:@"yyyy年MM月dd日 HH:mm:ss"];
    NSString *dateLoca = [NSString stringWithFormat:@"%@",second];
    NSTimeInterval time=[dateLoca doubleValue];
    NSDate *detaildate=[NSDate dateWithTimeIntervalSince1970:time];
    NSString *timestr = [formatter stringFromDate:detaildate];
    return timestr;
}

NSDate使用大全