Luke 3:1 "In the fifteenth year of the reign of Tiberius Caesar" – convention for counting years (NOT what the actual A.D. year was)

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1In the fifteenth year of the reign of Tiberius Caesar, Pontius Pilate
being governor of Judea, and Herod being tetrarch of Galilee, and his
brother Philip tetrarch of the region of Ituraea and Trachonitis, and
Lysanias tetrarch of Abilene,

This question is NOT what the A.D. year was but about the counting convention for years when mentioning reign of a ruler.

1. "the first year" – is that using the Julian calendar or Jewish calendar?
   I’d guess he’d be using Jewish years – Tishri-1 to Tishri-1
2. Partial year – regardless of Jewish vs Julian, would Luke count a partial year as "the first year"?
   If Tiberius assumed power 1 day before the start of new year, would that year (with one day left) be counted as "the first year"?