Halley's orbital period over the last 3 centuries has been between 75-76 years, although it has varied between 74-79 years since 240 BC [REF].
Halley has probably been in its current orbit for 16,000-200,000 years, although it is not possible to numerically integrate its orbit for more than a few tens of apparitions, and close approaches before 837 AD can only be verified from recorded observations [REF].