Anno Mundi (Latin for "in the year of the world"), the phrase "First of year, First of man" refers to the Jewish calendar system  the creation of the world and the first human.
Theophilus presents a detailed chronology "from the foundation of the world" to emperor Marcus Aurelius. His chronology begins with the biblical first man Adam through to emperor Marcus Aurelius, in whose reign Theophilus lived. The chronology puts the creation of the world at about 5529 BCE: "All the years from the creation of the world amount to a total of 5,698 years." No mention of Jesus is made in his chronology.
The "first of year" in this context likely refers to Rosh Hashanah, the Jewish New Year, which is considered the anniversary of creation, including the creation of Adam, the "first of man."It’s when the Anno Mundi year increments (e.g., the current Jewish year as of August 2025 is 5786 AM).Adam, marking the beginning of human history in the calendar system.