8 & 16 Bit Timers with Interrupts - Part 1 -The Basics on a Pic18F27K42
Understanding the basic idea of 8 & 16 bit timers with interrupts on a micro-controller.
Timers count from zero and include the zero, so any calculations should include a zero.
Though the timer registers only count up to 255, by using 256 on a calculator, it effectively includes our counting from zero.