Officially, it's 1 Jan (assuming you started work prior to that date) to 31 Dec, but because assessment is always done toward the end of the year, but not after the year, it will be 1 Jan to maybe Sep/Oct depending on when your assessment is done. If you are referring to how your PB is pro-rated, then it's based on the full year from 1 Jan (or whichever date you started work in that year) to 31 Dec, so just calculate accordingly.