Which Roman leader's calendar reform helped make January 1 the start of the year?

Julius Caesar's calendar reform helped establish January 1 as the start of the year.

The Julian calendar was introduced in 45 BCE after Caesar consulted the Alexandrian astronomer Sosigenes. It reorganized the Roman calendar by using a 365-day year with a leap day added every fourth year, bringing the civil calendar closer to the solar year.

January 1 had already been used as the beginning of the Roman consular year since 153 BCE. Caesar's reform preserved that administrative starting point and made it the beginning of the reformed calendar year.

The Julian calendar later drifted slightly because its average year was a little too long. Pope Gregory XIII's Gregorian reform in 1582 corrected that drift, but it retained January 1 as New Year's Day. Most countries eventually adopted the Gregorian calendar for civil use.