其实这题就是将第二个时间-第一个时间，小于0的补全就A了
代码如下：
var x,y,k:string;l1,l2,x1,x2,x3,y1,y2,y3:longint;
beginreadln(x);readln(y);l1:=pos(':',x);l2:=pos(':',y);k:=copy(x,1,2); val(k,x1);k:=copy(x,l1+1,2); val(k,y1);k:=copy(y,1,2); val(k,x2);k:=copy(y,l2+1,2); val(k,y2);x3:=x1-x2;y3:=y1-y2;if y3<0 thenbeginy3:=y3+60;x3:=x3-1;end;if x3<0 thenbeginx3:=x3+24;end;if x3<10 thenbeginwrite('0',x3,':');if y3<10 then write('0',y3) else write(y3);endelseif y3<10 thenbeginwrite(x3,':0',y3);endelse write(x3,':',y3);
end.