按要求一步步做就好

import java.util.Scanner;

public class dateAdd {

	public static void main(String args[]){

		Scanner input=new Scanner(System.in);

		String s1=input.next();

		String s2=input.next();

		int s11=Integer.parseInt(s1.substring(0, 2));

		int s12=Integer.parseInt(s1.substring(3, 5));

		int s13=Integer.parseInt(s1.substring(6, 8));

		int s21=Integer.parseInt(s2.substring(0, 2));

		int s22=Integer.parseInt(s2.substring(3, 5));

		int s23=Integer.parseInt(s2.substring(6, 8));

		int s33=(s13+s23)%60;

		int s32=((s13+s23)/60+s12+s22)%60;

		int s31=((s12+s22)/60+s11+s21)%24;

		String str=String.format("%02d", s31)+':'+String.format("%02d", s32)+':'+String.format("%02d", s33);

		System.out.println(str);

	}

}