"What's wrong with an external clock IC?"

Nothing at all, other than Dobrica George has been talking about using an internal timer all this time in this thread an his other recent thread.

"It's seems your way is to complicated for all of the type of 8051 derivatives."

Simply adding a handful of instructions, really? Perhaps it was the lengthy excerpts describing how the 8051 core works that threw you off.

Vanilla 8051, 8051 derivative, or non-8051 MCU, it's all the same to me -- I read and understand the architecture, the nuances, the fine print, and then write code to make it all work. The only curve ball some of the derivatives throw at you in this case are that those with reduced-cycle count instructions may need an additional NOP or two before you set the TRn bit again. But then if you are using one of these derivatives, you've read up on its little nuances and how that affects all of your firmware design, not just the timer stuff, right? ;-)