辛 is the 8th of the 10 Heavenly Stems (
天干),
未 is the 8th of the 12 Earthly Branches (
地支) — together they're year 8 of the 60-year
干支 cycle still used today for calendrical and astrological reckoning.
未 also corresponds to the Goat (
羊) zodiac, so 1991 was a Goat year.