If your birthday is late in the year, count yourself lucky - you have a better chance of achieving senior school qualifications.

A prize-winning study by University of Canterbury economists Asaad Ali and Andrea Menclova has found that your chances of achieving University Entrance (UE) increase by about 5 per cent for every extra month you spent in Years 0 and 1.

Children born in June have the highest chances of achieving UE because many schools put them into Year 0 for the rest of the year in which they turn 5, and then give them a whole Year 1 starting the next February.

Children born in May have the lowest chances of later achievement because they often go straight into Year 2 the next February, giving them up to about half a year less time in primary school than children born a month later.

The unique study was made possible by the New Zealand system in which most children start school as soon as they turn 5.

In most other developed countries, children all start school at the start of the next school year after they turn the required age, so they all receive the same amount of primary schooling.

However similar findings to the NZ study have been found for the Netherlands, where most children start school as soon as they turn 4.

The paper uses Statistics NZ's integrated data infrastructure (IDI), which allows researchers to trace individuals anonymously through multiple data sets. It won a Statistics NZ prize for "best use of official statistics" at this year's Association of Economists conference.

Czech-born Menclova is an associate professor of economics at Canterbury and Ali is a doctoral student.

They did not have data on when children actually started school, so their calculations are based on the time that they potentially started, and hence the amount of time they potentially spent in primary school.

They found that, for NZ-born children who left secondary school between 2009 and 2016, every extra month of potential time in primary school lifted their chances of achieving the National Certificate of Educational Achievement (NCEA) Level 1 by 2 per cent - after allowing for decile and other factors.

Each extra month also lifted their chances of achieving NCEA Level 2 by 4 per cent, NCEA Level 3 by 6 per cent and UE by 5 per cent.

At the extreme, a child born in June could get 6.6 more months in primary school than a child born in May.

So in theory, if every extra month increases your chances of getting NCEA Level 1 by 2 per cent, the June-born child has 6.6 times that advantage over a child born in May - increasing their chances of achieving Level 1 by about 13 per cent, Level 2 by about 26 per cent, Level 3 by about 40 per cent, and UE by about 33 per cent.

However Auckland Primary Principals' Association president Heath McNeil said schools were free to set their own policies about when children were classified as Year 0 or Year 1.

He said the cutoff for Ministry of Education funding was July 1. Five-year-olds starting before July 1 are classed as Year 1 and those starting after that date are classed as Year 0 in the rest of that year for funding purposes.

"So some schools use that cutoff, but many others use their own dates between May and June," he said. "Others use the end of Term 1 as their cutoff."

In practice, many schools run composite classes with children from different years grouped by their ability for particular subjects such as reading or maths.

Although the Canterbury analysis controlled for decile, McNeil said many other factors would also affect NCEA achievement rates.

"You'd have to look at other factors, not just the length of time that students spend in school," he said.

"When you look internationally, in some countries children spend far more hours in school than in New Zealand, yet we still have achievement levels that are greater than their achievement levels, so that almost contradicts the time at school argument."