Jeffrey Paul Delisle was given a 20-year sentence after pleading guilty to passing secrets to the Russians for 4 1/2 years.
But the judge presiding over his case ruled that when time already served is taken into account, Delisle's sentence is 18 years and five months.
The parole board says Delisle becomes eligible for unescorted temporary leaves in March 2016 and day parole in September 2018.
He is eligible for full parole a year later.
A decision on whether Delisle would be granted parole is made by the board after a review of his case or a hearing.