It's a simple process.
I will attach some screen captures to show the steps I took in this example prepared for you.
In the case of historic NZ B. D. & M. records, when you do a search online, you are only offered a year of entry, and a registration number. When you see an interesting result, you can limit your search to that year, then drill down through a number of steps.
I start with June that year, to see if registered in first or last half, I then try March, or September, depending upon the first result, then reduce the size [gap] of the steps, until I know the date the entry becomes active.
Remember occasionaly the registration is retrospective, so the registration year may not be the year of birth.
Exercise example attached.
Happy hunting.
