At 24MHz crystal the 8051 can only count up to 1MHz input frequency.
So for up to 1.3GHz you need a prescaler of minimal 1300.


And on the timer overflow interrupt feel free to expand the counter value with so many bytes as you want (e.g. on using DJNZ instructions).


Please watch, for accuracy of 5 digits and above, special care must be taken on the accuracy of the crystal on the 8051, since its the reference time base.


Peter