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Paul,
The ephemeris programs all compute comets using a set of orbital elements describing an ellipse which is really only valid for the current orbit. On the longer-lived comets the elements are updated from time to time so people can predict where to find them. At best, only an approximation valid for a short term.
The big problem with a close approach to a planet - as happened in the 9th century - is the orbit would have been totally changed - so much so it’s impossible to make any assumptions about appearances prior to that date. The period, eccentricity and the perihelion position would almost certainly be totally changed so any meaningful prediction is impossible prior to then.
The same effect is exploited by NASA to “slingshot” spacecraft to the outer planets.
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