;each second do the following:
   mov  a,Seconds
   add  a,#1
   daa
   cjne a,#60h,SecondsDone
   mov  a,Minutes
   add  a,#1
   daa

;   etc. - by now you should've get the idea

   mov  a,#0         ;clear the seconds
SecondsDone:
   mov  Seconds,a
   mov  P1,a