Which Roman ruler introduced the Julian calendar in 46 BCE?

Julius Caesar introduced the Julian calendar in 46 BCE.

The Roman republican calendar had drifted out of alignment with the solar year because its system of intercalary months was inconsistently managed. Caesar consulted the Alexandrian astronomer Sosigenes and reformed the calendar so that ordinary years had 365 days, with a leap day added every fourth year.

To correct the accumulated error, the year 46 BCE was made unusually long by inserting extra days. The Julian calendar then began operating in 45 BCE. Its average year was close to the solar year, but slightly too long, causing a gradual drift over many centuries.

The Gregorian calendar, introduced in 1582 under Pope Gregory XIII, corrected that remaining error by changing the leap-year rules. The Julian calendar continued in some religious and regional contexts, so dates may differ between Julian and Gregorian records.