Original: St. Leo I (440 461): 21 years, 1 month, and 13 days.
Simplified: Leo XIII (1878 1903): 25 years, 5 months and 1 day (9,281 days). Adrian I (772 795): 23 years, 10 months and 25 days (8,729 days). St. Sylvester I (314 335): 21 years, 11 months and 1 day (8,005 days).