飞行时长：(时差1+时差2) / 2

时差：两次互相飞的终点时间减去起始时间之差

import java.util.*;

public class Main {
    static Scanner sc = new Scanner(System.in);

    public static void main(String[] args) {

        int n = Integer.parseInt(sc.nextLine());
        while (n -- > 0) {
            int time = (getTime() + getTime()) / 2;
            int hour = time / 3600, minute = time % 3600 / 60, second = time % 60;
            System.out.printf("%02d:%02d:%02d\n", hour, minute, second);
        }
    }

    static int getSeconds(int h, int m, int s) {
        return s + m * 60 + h * 3600;
    }

    static int getTime() {
        String[] str = sc.nextLine().split(" ");
        int d = 0;
        if (str.length != 2) d = str[2].charAt(2) - '0'; 

        int[] arr = new int[2];
        for (int i = 0; i < 2; i++) {
            String[] string = str[i].split(":");
            int h = toInt(string[0]), m = toInt(string[1]), s = toInt(string[2]);
            arr[i] = getSeconds(h, m, s);
        }

        return arr[1] - arr[0] + d * 24 * 3600;
    }

    static int toInt(String str) {
        return Integer.parseInt(str);
    }
}