Some people have worked a pattern out, which maybe down to chance. Before the removal of the 99 rule, I was working on a pattern that was reasonably accurate for AM/PM guesses, I was talking to Dragon at the time if he remembers.
This idea went 50/50 when they removed the 99 day rule, but again I can't tell you if my 'system' was a system or just a chain of lucky guesses.