MainLoop:
A0 = 0
IF PORTA.5 = 1 THEN A0 = A0 + 1
A1 = A0
IF A1 = 600 THEN AddHourTenth
Gosub DisplayRefresh
Goto MainLoop

It looks like every time you hit MainLoop, you reset A0 to zero. Subsequently, you put the A0 value into A1. This gets repeated every loop. The most A1 can increment to is 1. Also you have a goto statement to a sub loop, with a return at the end. Either have a gosub with a return or a goto with a follow up goto. Otherwise there will be stack problems.

Ron