Numerologists have different opinions when it comes to calculating the Transits (Essence) and the Personal Year. Calculating Personal Years causes the most turf wars.
I don't ram my opinions down somebody's throat because what's important, is that your numerology principles prove true for you and your clients.
That said, my numeric perspective is your new Personal Year begins on January 01; however, it is to be presumed that the newly arrived energy takes time to come to full power and therefore, the yearly Cycle doesn't get underway until the earliest Personal Month 1. Up until then, you plod through matters in connection with your previous Personal Year.
The exception to this guideline is the Personal Year 9, an 'ending' cycle which gets underway immediately on January 01.
To give an example, on January 01, if your Personal Year is 7, you will continue to press ahead with matters from your Personal Year 6 (housing/repairs, health, marriage, parents, children). Once February arrives, the Personal Year 6 energy starts to grow faint, and in consequence, during February, you are confronted with new themes while continuing to sort out old matters from your Personal Year 6.
In next to no time, it's March, and you instantly become aware of new situations that are suggestive of 7 (fatigue, insomnia, illness/injury, depression, deceit, moodiness, separation/divorce, taxes, a growing concern with financial matters, secreted information, legal issues, debts, health). In April, and for the remainder of the year, you are fiercely contending with situations symptomatic of your Personal Year 7.
Most of all, keep in mind, for all Personal Years, September of the current year through to March of the following year is likely when you'll see the results of your efforts during the year.
To sum it up, whether you accept that Personal Years start on January 01, or your birthday, be careful about placing too much credence on Personal Years and not developing your ability to interpret the overlaying Life Cycles and Letter and Number Transits (Essence). Come what may, what numerologists do agree on is that Personal Years will help to give you a general idea of the trends for any given calendar year.