字符串日期與秒數互轉

- (void)viewDidLoad {
    [super viewDidLoad];
    NSString *str = @"2016-09-13 21:47:06";
    NSTimeInterval seconds = [self dateStrToSeconds:str];
    NSLog(@"%f",seconds);
    NSLog(@"%@",[self secondsToDateStr:seconds]);
}

//字符串日期轉秒數
-(NSTimeInterval)dateStrToSeconds:(NSString *)str
{
    NSDateFormatter *formatter = [[NSDateFormatter alloc]init];
    [formatter setDateFormat:@"YYYY-MM-dd HH:mm:ss"];
    NSDate *date = [formatter dateFromString:str];
    NSTimeInterval seconds = [date timeIntervalSince1970];
    return seconds;
}

//秒數轉字符串日期
-(NSString *)secondsToDateStr:(NSTimeInterval)seconds
{
    NSDate *date = [NSDate dateWithTimeIntervalSince1970:seconds];
    NSDateFormatter *formatter = [[NSDateFormatter alloc]init];
    [formatter setDateFormat:@"YYYY-MM-dd HH:mm:ss"];
    return [formatter stringFromDate:date];

}

2016-09-14 21:35:44.348 時間轉換[1436:129333] 1473774426.000000
2016-09-14 21:35:44.349 時間轉換[1436:129333] 2016-09-13 21:47:06

總結：都得轉爲NSDate作爲中介