In the 8051, the timer counts up and when it overflows it clocks the uart or fires the interrupt.

so, you have timer = 0-time. 0 being the value when it overflows.

say you want the timer to interrupt every 16 clocks.

the timer gets loaded with 0-16 = 240 (assuming a 8 bit timer).
So the timer counts 240,241,242....255,0 (interrupt!).

This is explained in the bible. So do a bit of reading on the timers and all shall be revealed.