You have to realize that this watch may have quite big accuracy error. It depends what's the purpose of the watch/clock/timer. Some accuracy might be enough and some may require external device/clock of high stability and low tollerance of resonator.


There is many exmples how to do that, but if you are looking to make accurate home clock on 8051 only I would not go there withouth external independent circuit... or if Dallas makes something built-in then with some Dallas controllers.


I made some darkroom timer for enlarger on AT89Cxxxx but that accuracy is pretty enough for photography even in professional studio. For long term time measurement I do not think simple circuit would work well.