The leap day in the year 2000 has caused a headache for Canberra courts, after a girl born on February 29 of that year was charged with a crime committed exactly 18 calendar years later.

Key points: Alleged offender was born February 29, 2000

Alleged offender was born February 29, 2000 Charges stem from incident on February 28, 2018

Charges stem from incident on February 28, 2018 Supreme Court determines she was 17 at time of alleged offences

The girl, who cannot be named for legal reasons, was being treated as an adult over offences allegedly committed on February 28, 2018.

As there was no February 29 in 2018, the ACT Children's Court initially found the girl had turned 18 on the day of the crime, and should be dealt with in the adult court system.

But the girl's lawyers applied for a review, arguing she should not be treated as an adult until March 1, 2018.

Her lawyers said that as a matter of logic a person born on February 29, 2000 would not be 18 years old on February 28, 2018, in much the same way as a person born on February 2, 2000 would not be 18 on February 1, 2018.

But the Director of Public Prosecutions (DPP), which sought to have the girl dealt with in the ACT Magistrates Court, argued that reasoning was "simplistic".

Summarising the DPP's submission, Supreme Court associate justice Verity McWilliam said: "the director submitted that a person's age can be determined by reference to the definitions of 'calendar month' and 'month'."

"Each year that the plaintiff has aged since 29 February 2000 is measured by the passage of 12 calendar months.

"Substituting the definition of 'month', because there is no corresponding day three out of every four years, the 12th month of every common year the plaintiff has been alive, concludes on 28 February."

However, associate justice McWilliam disagreed with that argument, finding the girl should be treated as a child in relation to the offence in question.

She said that a person did not reach a given age until their birthday had begun or passed.

"It follows that on the proper construction of [the law in question], on 28 February 2018, the plaintiff was not yet an adult, being someone who was 'at least 18 years old', because she had not yet reached the beginning of the anniversary of her birth.