iOS 时间戳转换成时间及两个时间差

//时间戳
NSString *beginTimestamp = @"1489586485";
NSString *endTimestamp = @"1489811147";

    NSDateFormatter* formatter = [[NSDateFormatter alloc] init];
    [formatter setDateStyle:NSDateFormatterMediumStyle];
    [formatter setTimeStyle:NSDateFormatterShortStyle];
    [formatter setDateFormat:@"yyyy-MM-dd-HH:MM:ss"];//@"yyyy-MM-dd-HHMMss"
    
    NSDate* date = [NSDate dateWithTimeIntervalSince1970:[beginTimestamp doubleValue]];
    NSString *dateString = [formatter stringFromDate:date];
    NSLog(@"开始时间: %@", dateString);
    
    NSDate *date2 = [NSDate dateWithTimeIntervalSince1970:[endTimestamp doubleValue]];
    NSString *dateString2 = [formatter stringFromDate:date2];
    NSLog(@"结束时间: %@", dateString2);
    
    NSTimeInterval seconds = [date2 timeIntervalSinceDate:date];
    NSLog(@"两个时间相隔：%f", seconds);
    
    NSTimeInterval seconds = [date2 timeIntervalSinceDate:date];
    NSLog(@"两个时间相隔：%f", seconds);